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

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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)