Why is Mastodon not effective for all potential social media uses?
For the same reason Twitter can't be Instagram. Or why Facebook can't be TikTok. Or why YouTube can't be Reddit.
Mastodon is excellent for microblogging, but the moment we try to shoehorn other purposes into Mastodon, Mastodon becomes less effective at microblogging.
If you want a real world scenario for why this is, just look at Instagram when Meta moves it away from its core function.