Isn't offering an I.O.U. exactly what a credit card is supposed to do. They are doing such a bad job for merchants he would rather run the I.O.U. service himself than use their product. It seems like a business ripe for disruption - who really likes Visa/MC/AMEX?
It's about risk. Visa/MC/AMEX deals with people worldwide, which includes some crooks and thieves that are trying to scam them (and will continue to do so, under multiple identities). The coffee shop only deals with customers it sees personally, so it can keep people from scamming them multiple times, or for large amounts.
Any business that tries to disrupt Visa/MC/AMEX is going to have to be able to handle and reject large-scale fraud.
To answer your question literally: Congressmen like credit card companies because of their large campaign contributions. Anyone hoping to disrupt will have to overcome the fact that the rules of the game are being written against them. Paypal and Square seem to be doing alright though. Google wants to get into the market as well.
Not to mention all this Bitcoin stuff we've been hearing about recently. If it actually takes off there's potential in setting up physical systems/cards to facilitate point-of-sale bitcoin transactions in physical space.
