[CyberFonic]

PayPal are enjoying the "first mover" advantage. A lot of sites allow you to use PayPal so it becomes convenient. Personally, I am rather wary of providing my credit card details to any random site's shopping cart.

Having said that, I don't use PayPal when transacting with parties that I trust. In financial terms PayPal only captures about 15% or my business.

To answer your last question: It is a very large workload to establish relationships with banks and credit card companies in order to create a viable alternative to PayPal and then you still need to build up the user base. It can be done, but will take considerable resources and time to accomplish.