Because it's a big market companies try things first in the US - if they can't make money on something there they may not make money on it at all. It means they get things first, but it also means they're guinea pigs for the things that don't work or are withdrawn because they're not profitable. By the time things that start in the US move abroad they work ... and sometimes the price has come down because of all the people in the US who have already invested in it.
It's not all bad! And if the kindle was easily available in the uk I probably would have bought it, because it's so widely marketed. But as it is I bought a sony and love it.
