I think that's because in Europe an automatic gearbox has always been a pretty expensive option on most cars while in the US there are a lot of (non luxury) cars sold with an automatic as standard equipment.
Take a look at Chrysler, automatics standard across the line*. A Dodge Challenger; the base model is automatic only. You need the more expensive versions if you want a manual. Those are just two examples of course and there are probably many exceptions. But generally and automatic gearbox is much more common as standard equipment in the US.
That's completely the other way around when compared to Europe.
*A 4-speed on the Chrysler 200. A 4-speed in 2011, really??