Because he's been fucking them for 6 months and they haven't left.
I did, and some others have. But most people are still sitting there on his platform, whining about how shitty it is, while keeping him in power by staying.
Would you rather have 10 users paying nothing but taking up server space and bandwidth? Or would you prefer 1 paying user and 9 people leaving? In the short term, option B is a win-win.
And if that leads to Twitter becoming a dead husk of itself, oh well, that's a problem for next quarter