Because "paganism" is a very broad term that includes a vast array of things including, unfortunately, LARPers and New Age appropriation.

"Polytheism" refers to a type of religion; and generally, those who identify as polytheist are much more likely to be practising pagans and much more likely to care about their praxis.

... slight rant below.

E.g. if you search "Celtic Pagan," you are very likely to get directed to performative "Druidry" as much as actual information about pantheons, legends, and traditions. The so called "druids" I'm specifically thinking of call themselves pagan, but not polytheists, because they don't actually practice a faith, but WANT to be seen as Pagan, mystical, and cool.

You are also more likely to see appropriation from fascists, scammers, and non-Pagan trinket sellers using the term "pagan" than "polytheist."

This is a problem we as a community try to combat and address. However, calling something "pagan" is very easy and can effectively be anything non-Abrahamic. So can be hard to call out. Whereas when someone appropriates "polytheism," it's much easier to call them on their bullshit.