Sidestepping issues about what "monotheist" and "polytheist" actually mean, I would suggest that it's a historically contingent phenomenon based on the adoption of Abrahamic religion by a couple of prestige civilisations. Where monotheistic faiths have cultural and social hegemony, it will often gain converts, and where polytheistic beliefs have cultural and social hegemony, monotheists will often deconvert.
The monotheistic Jews were under such threat of creeping Hellenisation that the zealots started a civil war over it.
The Nestorian Church in China declined and was absorbed back into traditional Chinese society, same for most of the Kaifeng Jews and to a lesser extent Chinese Muslims.
There was mass conversion of Christians back to Buddhism/Shinto under persecution in Tokugawa Japan, and even the Hidden Christians adopted traditional Japanese polytheistic aspects over time.
And it's not clear to me whether Hinduism is losing or gaining converts in post-independence India (maybe the former, I genuinely DK but just putting it out there).
There could be more examples but those I came up with off the top of my head.
One reason behind this is that monotheistic religions tend to be a bit more “personal” than polytheistic ones, which makes sense- polytheistic religions tend to be more about the gods interact with one another, while monotheistic ones of course focus on how God interacts with human beings. Monotheist Religion therefore gives human beings more purpose, and more reason to be developed.
Have you heard of Bhakti?
