I can't think of a single way that would benefit them, so if you don't mind I'm going to flip the question around.
Why should Twitter implement activitypub?
They can't. It would destroy their moat and since they are already unprofitable it would be the end of the company.
Ah, that magical word, “just”…
Twitter, facebook, youtube treat their users as capital. their value is that the users can't move to simething else. If they would implement activitypub they would effectivly burn money