[makeitdouble]

I don't know if we'd call that a "network effect", but there is a (vicious?) cycle cementing PayPal's position.

More often than not sites will get different prices based on the transaction volume, so having a number of small minor payment options is of little benefit, except for very specific cases like Alipay.

Then if volume is transaction an important consideration, going with the market leader is the most reasonable choice.

I'd assume that's the logic Nintendo applied for its online store for instance, making PayPal the only alternative option to standard payment systems.