There are a million reasons. First, traffic in Europe does not *tend* to be as bad as the US. Most US workers are stuck in stop and start traffic for an hour + every day of their working life. We don't have effective mass transit in any large quantity, so we are forced to deal with horrible traffic.
Since most now have learned and grown up with auto's, our driving habits (which are terrible) are conducive ONLY to driving an automatic. Jackrabbit starts and stops are not a driving style with manuals, thus people get automatics to drive like everyone else.
Manuals have been better with small "underpowered" cars. Now that the engines make more power, that difference is not so great. BUT, as with any debate on manual vs. auto, the manual costs less, weighs less, the manual driver is able to anticipate gear changes, not wait on the response of an automatic, compression braking is better, and longevity is greater.
Unfortunately, BECAUSE everyone drives like an idiot here anymore, our fuel economy rating testing has changed to reflect the poor driving habits. They are actually a "worst case" scenario, not what the car is capable of like they were 15 years ago. So people like myself get 45MPG in normal driving, when the car is rated for 35. This of course, doesn't make people think about their poor driving habits, so nothing changes for the better. This is why you will often now see better "city" driving numbers for the automatics than manuals, but again, those numbers are bettered by a good driver. An automatic can not physically ever achieve the efficiency of a manual, since they require far more power to operate.