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

[Tartarus Sauce]

The Abrahamic monotheistic model is one that actively seeks to convert the uninitiated since they are "world religions" that adopt a universalist worldview. Most polytheistic traditions have localized, parochial origins and were thus generally (but not always) far more accommodating in regards to the religious practices of neighboring peoples.

There's simply more of a missionary mindset hardcoded into the baseline outlook of the likes of Christianity and Islam than most polytheistic faiths.