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.