This is because consumers love PayPal. It means I am not giving my credit card to a potentially shady person and that I have a way to cancel the service. I also have 1 place where I can see all my subscriptions, so I don't keep subscribed to services I don't use but forgot about.
In general, if there are two equivalent options and one takes PayPal and the other doesn't, I am going with the one that takes PayPal.