Good point, and also the EU has >60% greater population to cover that smaller landmass (it is the EU, not Europe as a whole by the way).I'd say it's mostly land mass. Europe on a whole is less then half the size of America. So because of that you have less operating expenses and can pass on that savings to the consumer. It also means you can upgrade your network faster because you need less towers and that also mean's it's cheaper to do so (operating expenses again) which probably explains why the network was a few years ahead; Although it's basically at the same point right now since LTE isn't out yet.
It's certainly a combination of factors, but there's still plenty of room for improvement. Look at the solid network and cheap deals Australian's get - especially given their remote landmass and tiny customer base (the reason CDMA stuck around so long there is that it's efficiency offered better coverage in the 'bush').
Sorry all, think I've just bored myself to sleep too!