But then why not ask for it when registering?
Well, of course to decrease friction and avoid turning people away at the entrance. You get smacked in the face with the request for your phone, once you've followed some interesting people, interacted with the site a bit and will form some attachment. Then you are more likely to enter your phone to keep using it.
There's a psychological effect of feeling more strongly about retaining a given amount of already owned value compared to gaining an equivalent amount of stuff you don't have yet.
You wouldn't pay with your phone number to get a Twitter account, but you would pay with it to keep an account you already have and started using.
Dark patterns.