AFAICT, even though monotheism is less fundamental, it has outperformed polytheism in the past millennia or so, so the vast majority of religious people in the world (at least outside India) can be considered monotheistic. Even the “official” Hindu belief is that all the gods are emanations of one true god, which doesn’t seem to necessarily be the historical view. What about monotheism makes it so preferable to polytheism? I would guess that polytheism makes more logical sense from a pre-scientific POV, and most of the change happened before modern scientific understanding. Secondarily, why isn’t dualism more common? It seems to be more coherent than monotheism and it doesn’t have the same theological problems such as the problem of evil.
A lot of polytheistic religions are henotheistic, acknowledging worshiping solely a supreme deity but acknowledging the existence of others. early Judaism is believed to have been henotheistic, but later shedded the other dieties, which leads to the most important reason IMO that polytheism and henotheism largely subsided after antiquity: exclusivity. Abrahamic faiths are very cut and dry on the definition of their believers, which makes it very important for the state to support and propagate, as it gives believers specific rights and privileges. Constantine successfully was able to redefine the meaning of “Roman”, those who were not followers of the Great Church were not Roman and could not be Roman. One Church for One Emperor.
Now from a historical context: the Jewish population during the early Roman empire was significantly larger than many are led to believe, and hence the public also had significantly large awareness of the Jewish religion than we are lead to believe. Jews werent just located in Judaea, but they had large populations in pretty much every city and town in the Roman East. In Colossae, the public worshipped the Abrahamic angels. In large parts of Anatolia thr public worshipped Zeus Hypistus, basically Greek Yahweh. Greek Roman literature and the Bible also both mention the class of believers known as “God-fearers”, followers of Judaism who were not Jews because of circumcision or other beliefs who eventually became Christian due to the Paul’s teaching of the New Covenant. Thus from this high general awareness the Church could rapidly expand and reach a high level of organisation. Furthermore Neoplatonists even pushed towards a philosophical monotheism with their belief in the Monad, so the educated class even had basic awareness of monotheism as well.
For Islam, many people use Jizya as a reason, but i personally disagree especially in the . Islam propogated because of massive changes in agricultural practices in the Near East and Mediterranean that were spread by the Arab conquests. Areas that had never been cultivated or inefficiently cultivated were settled, and the settlers converted to the faith of their masters. The perfect example of this is Bengal, where the the population became Muslim because of the area essentially being an empty jungle before the Sultans began clearing the land. A similar process happened in Sicily, where the Arab conquest restored the island to being a breadbasket and of course it rapidly became Muslim in the course of 2 generations.