Probably the same reason why PayPal doesn't take Skrill as a funding method, Them being a competition and in the exact same market PayPal is in right now, I also know they discourage using BTC in Ebay as well and only using PayPal, I guess that might change in the future though when bitcoin goes mainstream.
Since Bitcoin is being well known worldwide and becoming more popular nowadays, I think it might surpass PayPal someday. Many people find it more convenient rather than using PayPal as a mode of payment.
Yes, paypal gives too many problems to its users, it is very strict with its policies and many users are not happy about that. The reason why many people will be forced to use paypal is because there is no alternative for it, as we now have bitcoin I think in few years to come bitcoin will completely take the place of paypal in the entire world.