Because the short-term equation is easy:
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
Dont forget that other favorite “No you can’t have more hardware, these were fine in 2006 and they’re fine now! Just delete some old stuff like you IT people always do”