Because law enforcement (at least in the US) is woefully bad at their jobs and don't care. I have first-hand experience with this when I had a close friend who was being harrassed by someone local (we knew who it was) and LEO refused to lift a finger. I've talked about it multiple times before on HN [0][1][2][3] but even with this guy graduating to physical actions (keying the friend's car, following/stalking them) the police did nothing.
So if our only options are "Have the police do nothing" or "Have a private corp decide to not do business with a company anymore" then I know which one I'm picking, which one will actually help reduce harm. Even if they spin up under a new name they will lose users along the way. Deplatforming works.
[0] https://news.ycombinator.com/item?id=32521976
[1] https://news.ycombinator.com/item?id=31610876