This paper is very helpful in explaining why some Nordic countries did and others didn't join NATO:
https://apps.dtic.mil/dtic/tr/fulltext/u2/1046807.pdf
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"At the conclusion of the Second World War, Sweden had remained both neutral and out of war for over 150 years, and most Swedes considered the latter a direct outcome of the former.116 The policy had been viewed as a success for so long that many in Sweden believed that “a good Swede is a neutral Swede.” 117 Indeed, Christian Günther, the Swedish foreign minister during the war, claimed in his last official speech that “Sweden succeeded in what everybody had wished for, that is to avoid war,” and that Swedes were even “able to help [their brother people] in so far as was possible.” 118 Minister Unden, who probably had the greatest impact of all on Sweden’s decision not to join NATO, argued that neutrality was so popular that public opinion would not allow any change from it.119...
The same paper noted that the US and the UK tried contrasting methods for inducing Sweden to join NATO:
"One of the reasons that Sweden ultimately chose isolated neutrality was that American and British leaders failed to influence them to do otherwise. This failure was caused in large part by American and British disagreement on how to best handle the reluctant Swedes, which resulted in the two countries choosing opposite strategies. The United States decided to employ a heavy hand with the Swedes and began “rebuffing feelers from Sweden on informal staff discussions, stalling export licenses for military
technology to Sweden, removing Swedish ports from U.S. naval units’ visit schedules to Scandinavia, and reiterating the fact that, unless Sweden abandoned neutrality, no last
minute help would be forthcoming from the U.S. in an emergency.” 131 Furthermore, the United States told Sweden that it could not expect to buy American military supplies if it remained neutral, and in 1948 the U.S. ambassador to Sweden, Freeman Matthews, and the U.S. Counsellor at the embassy, Hugh Cumming, told a group of Swedish
businessmen that in a war the United States would bomb Swedish industries “out of existence” in order to prevent their use by the Soviets.132 The result of this policy was
that the Swedish military, afraid of being isolated and without American military equipment, came to believe that their government “was making a serious mistake” and
consequently began “pulling all possible strings within and outside the Swedish government to make it change its mind” regarding neutrality.133
"The British, on the other hand, preferred what Robin Hankey, the head of the British Foreign Office’s Northern Department, called “crafty diplomacy.” 134 The British
thought that the United States had pressed Sweden too hard and that this was counterproductive.135 The British, in turn, took a more agreeable stance towards Sweden and
began selling Vampire jet aircraft to Stockholm, which completely undermined the American strategy.136 Ambassador Matthews lamented in response that the “reasonably
good chances of getting Sweden into [the] western camp are being sacrificed by [the] short run interest of [the] British aircraft industry and British ignorance of Swedish
psychology,” and that if the British did not cancel the contract then “we may just as well forget any thought of winning Sweden from neutrality.” 137 At the urging of London, the
United States eventually gave up trying to influence the Swedes by withholding supplies, and this left the Americans without a strategy at all. This deficit, combined with what the Foreign Office interpreted as a lack of urgency from Washington regarding the formation of the Atlantic alliance, hamstrung the attempts by the British to convince Sweden to seek protection from the United States. 138 The U.S. position, in the end, was that only Greenland and Iceland were necessary for the Alliance because of their strategic value, and Sweden could remain outside if it preferred. Ultimately, Bevin and U.S. Secretary of State George Marshall gave up on Sweden and decided not to invite the Swedes to the negotiations that led to the formation of NATO, thus signaling the final failure of U.S. and British policy to sway Sweden into the Alliance.139
"Although the Americans and British failed in this way, it is possible that they never could have succeeded in the first place as long as Sweden’s foreign minister was Östen Undén. Undén insisted throughout the period that there would be no change to Sweden’s foreign policy of neutrality, saying that “it would be disastrous… if Russia got
the impression that Sweden might be used as a base for attacks upon Russia.” 140 Consequently, after a speech on 4 February 1948, in which Undén confirmed his trust in
neutrality, Ambassador Matthews concluded that “any Swedish departure from neutrality must be over Undén’s dead body.” 141 The British conclusion was the same: no
fundamental changes in Swedish foreign policy would happen with Undén in charge.142 This realization directly led to U.S. and British decisions to proceed in creating the
Alliance without Sweden.143 Thus, Undén’s role in preventing Sweden from joining NATO as a founding member cannot be overstated.
"According to new research performed by Robert Dalsjö, one reason behind Undén’s determination to stay neutral was that by doing so Sweden could remain outside of any devastating initial nuclear exchange between the East and the West if the Cold War turned hot.144 This research indicates that the Swedish leaders understood that they would be forced into the fray if war erupted, and in such a conflict they would join the Western forces, but that they preferred to join the war after the initial salvos were fired.145 Evidence that this is true can be found in documents about previously secret arrangements that the Swedish and U.S. governments had made for the protection of both Sweden and the NATO Alliance in the event of war. Sweden had secretly agreed to allow the overflight of U.S. aircraft for bombing missions against the Soviet Union, had secretly made preparations to receive NATO forces and provide help to the NATO war effort, and had cooperated extensively in intelligence sharing with the West.146 This cooperation ultimately proved fruitful, as Sweden was rewarded with the ability to purchase American arms starting in 1952, when it became part of the Mutual Defense Assistance Act.147 The Soviet Union even knew about these secret Western ties, further decreasing the likelihood that Sweden could stay neutral in war.148 However, as has been noted, Swedish leaders viewed the possibility of avoiding inclusion in the Soviet Union’s initial nuclear attack plan as a convincing reason to remain outside the Atlantic alliance in peacetime. At the same time, Swedish preparations to work with NATO in the event of war might have acted to enhance the overall Western deterrence posture.
" An additional argument made by Swedish leaders for not participating in NATO’s creation centered on what became known as the “Finland Question.” That is the argument
that if Sweden aligned with the West, the Soviets would find it necessary to exert greater control over Finland in response.149 Finland becoming a Soviet satellite was not an acceptable outcome, so the Swedes argued both publicly and privately that they could not join the Alliance.150 This upheld what became known as the Nordic balance, the array of international relations that emerged in the Nordic region, with Denmark, Iceland, and Norway in the Western camp, Finland in the Soviet orbit, and Sweden neutral in between, thus balancing the system. Jakobson, however, has argued against the proponents of this idea, suggesting that the notion that Sweden stayed neutral for the sake of Finland “would make as much, or rather, as little sense [as] to claim that Finland fought to retain her independence in order to make it easier for Sweden to stay neutral.” 151 In any case, whether their purported concern for Finland’s status was sincere or not, the Swedes succeeded in using the argument to justify their neutrality to the Americans and British: the British Foreign Office eventually pointed to this argument themselves and even believed Soviet pressure on Finland was aimed primarily at keeping Sweden out of negotiations for an Atlantic alliance.152 Thus, the Finland question and the idea of a Nordic balance appear to have played an important role in Sweden’s decision not to join NATO regardless of how sincere the Swedish politicians were in invoking it. "
