Why did so many polytheistic people were converted into monotheism but the opposite way is so rare?

[RINO Tom]

I think it’s worth mentioning that - regardless of your opinion of them today - the Abrahamic faiths seemed a lot more enlightened and modern.  Germanic paganism taught that the world was held up by a tree … that’s a lot easier to disprove than the description in Genesis.  Whereas Hellenic paganism was effectively preaching gaining Earthly glory to affect one’s afterlife prospects, Christianity was promising slaves and women that no almighty God would care about such trivial, mortal things.  Traditional paganism taught that if you did [ritual to God X], you’d get [better outcome related to X].  The more interconnected the world became, it’s easy to see why monotheism caught on.

[RINO Tom]

^ I kind of came back here to add that "Hellenic paganism" was already in steep decline (at least from its original, belief-oriented version during the "Trojan War Era") by the time Christianity even became a thing, and it was largely ceremonial for many Romans by the time of Constantine.  Monotheism (and all post-Axial Age faiths) provided a sense of philosophical fulfillment that I think had been sorely lacking from a decadent Roman Empire likely since the days of the early Republic.