I have tried to find the answer to this but I cannot find it so I thought I would ask here.
Why did the United Kingdom have Hong Kong Island permanently seceded to them but only get a 99-year lease on the New Territories?
Would it not of made more sense to have the New Territories permanently seceded to the UK and made a permanent part of the Hong Kong colony.
The UK had Hong Kong island permanently ceded to them because they got it through war, e.g. they beat the Qing fair and square. Same with Kowloon.
The UK desire for the New Territories stemmed out of a desire to protect these two colonies, which were potentially threatened from the rapid expansion of other countries' interests in China, which were also generally 'leased', like Zhanjiang/Fort-Bayard to France and Qingdao/Kiautschou Bay to Germany.
The UK wanted the New Territories to act as a buffer zone against a landward attack on Hong Kong, but it wasn't willing to risk a war with the Qing (however much of a cakewalk it could have been) for it. 'Leasing' the territory, therefore, was a face-saving measure for both the Qing (who wouldn't be giving land away), and for the imperial powers (who wouldn't be seen as aggressors).
As for why 99 years? Well, in the words of Claude MacDonald, the chief negotiator, it was a length of time that was 'as good as forever'. Rather stupid in hindsight, but it's not his fault that he couldn't see how the New Territories would become a vital part of Hong Kong in the intervening period. And even in modern-day land contracts, it's extremely rare for land to just be given away in perpetuity - usually it's given for a period of 999 years. I'm sure people in 3014 will be cursing our lack of vision on this front...