Here's a semi-educated hypothesis: a lot of polytheism is essentially Just So stories and codified superstitions. Why does sea go up? Sea god says so. What brings the seasons? Persophone is going into Hades? Who brings our household luck? The statue in our room of our patron god. There is a lot of intrinsic charm in these beliefs but they don't really stand up to too much philosophical enquiry. You don't even have to blame the abrahamic faiths here: the Greco-Roman religious beliefs had long ossified into a set of rituals by the time it was displaced by Christianity, and Christianity itself was one of many competing Eastern religions that seemed to place a new god into a semi-monotheistic Head Deity position (two roman emperors, Elagabalus and Aurelian, were full on believers in monotheistic Sun Gods, with only the latter being actually successful in forming a fairly powerful religion that outlasted him, the cult of Sol Invictus)
Yeah.
Although many conversions to monotheism were done by force, there were other motives.
If you don't follow the Bible litteraly, it's much more feasible to concile science to a monotheist faith, in which there is an invisible God, than to a polytheist faith, in which gods are like human beings or like a mix between human beings and non-human animals.
This. Monotheism is just inherently more rational. This means it will have an appeal to a significant portion of the population that quietly rejects the pagan gods as ridiculous. The monotheistic stories are a lot less convoluted and lean heavily on a conception of natural moral law. While the parts of modern Protestant Christianity and Islam that focus on taking every detail literally get outsized media attention, this isn't the norm globally and those denominations generally didn't even exist in ancient times when the educated classes of Rome, Greece, Syria, etc. were converting.
