Because they are regulated as a payment processor. Same reg issue as the rest of us: FinCEN AML stuff regarding MTs and MSBs. FinCEN has effectively said you can't legally move cash. They're basically treating BTC like a foreign currency (which is what we want) but they're confusing the issue by not explicitly saying so, and then all the other gov agencies have their own versions of an opinion too, making it murky.
I've been saying for months that PayPal, at least at the exec level, is cautiously pro-BTC. Cautious in that they intend to not be at the forefront, since the shareholders would throw a fit, but they'll adopt it when the time comes (which may be sooner than later, if we pick up another Overstock-sized retailer or two, or if there's finally official word from the IRS etc. on what they want people to do with it). Musk himself is quietly watching. I wouldn't be surprised if he owns any, though he wouldn't say specifically.
It'll be interesting to see how he reacts to the news that rigs and general crypto gear is still being used for TOS violation emails. Clear case of breakdown in intra-company communication.