This is strange to me. I have used the IMDB forums in the past, not in recent years, but it always seemed like a friendly place, where you could learn things and discuss movies with other fans, both those who liked or hated a movie would chime in... Most of the people were nice, there were a few trolls here and there, but nothing major.
Since I have been out of the loop, maybe they did have big problems start happening with trolls and spammers. I don't know. But I still wonder if, as was commented above, if politics was part of the decision...since everything has become super polarized, and Hollywood, as a whole, has adopted a hard core political position, which probably meant a lot more politics was being injected (unnecessarily) into the discussions, by those arguing for and against Trump. Keep in mind I'm guessing here, I could be wrong.
If that is true, then only Hollywood has itself to blame, for becoming so hyper-partisan...
That can't be good for business either, not in the long run. You want to appeal to as many people as possible and want them to go see your movies, regardless of political leanings or affiliation. Even if you are talking about *God forbid* Trump supporters (I will humbly beg forgiveness, and say 3 hail mary's as penance for bringing up the devil himself, and his evil acolytes (which include me)...