Speculation: Europe has a lower risk management cost than the US when it comes to card transactions, making it more profitable.

Smartcards with PINs were ubiquitous much much sooner (it being a French idea did not help for US market penetration), which means today the magnetic stripe on our cards is little more than decoration. Seriously, I'm 29 and I've yet to see a single magstripe payment here (while it was daily when I went to the US).

Also, we pretty much only have debit cards (don't know why, but don't know why I would want a credit card). This is even less risk for both the network and the merchant and the bank, reducing issues for all links in the chain.

Or the fact that Europe has 100M extra people in it, and a lot of countries seldom use cash nowadays.