Paypal was the first to rise to the top, and has since benefited from the network effect. It's only been about 5 years since the average American started to be comfortable buying products online.
There were real competitors when PayPal was a younger company, but a lot of them went out of business because they didn't solve the fraud problem. It wasn't just a matter of adoption. PayPal hemorrhaged millions to fraud before they created the technical and people systems to make it manageable.
