You have to look at what businesses exaclty accept paypal. Most you will find are too small to have their own merchant solution. In other words they are lacking in options. The other factor is the type of business. For those selling tangible goods the chargeback rate is not nearly as high as it is for digital goods. It is easier for a merchant to prove they delivered something to the identity that bought it verses proving they granted access to something online. Paypal has in recent years taken a few steps to make it a bit more controlled for those dealing with chargeback claims and digital goods. That and the hardline approach of just not accepting some merchants who deal in certain digital goods, like bitcoin.
For credit cards, that's obvious, because everyone has them, to put it most simply. But, like the paypal users, the CC accepters that are not large enough to have an in house provider, an intermidiary that helps fight frivelous claims, their chargeback rates are likely higher for digital goods. They also have few options.
Gotta run, hope any of that makes any sense. ;p