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.
The burning question is why would one use Paypal for bitcoin?We needed Paypal,Skrill etc to send or receive money because we had no choice but with the advent of bitcoin,we dont need PayPAl anymore