I personally think the answer is just because America is much more conservative as a whole that European countries. Recall America was mostly populated by the Quakers, the group of traditionalist bible-literalists that were too biblical-strict by European Renascence standards. I think a lot of that conservative-ism is still ingrained into US, especially in places like the bible belt. Abroad, many countries still attack the US for being so conservative... liberal people in the US are very conservative compared to many other government parties across the world.
The first major affect that such religious-driven conservatism is that prostitution is legal in Germany because they don't create laws based on morality like Americans do, at least not to the same extent. Less risk of getting busted, people would do it for less. Further, if it's not illegal, more people are willing overall, which drives prices down. (Economics 101)
http://en.wikipedia.org/wiki/Prostitution_in_Germany
Also, laws aside, if they aren't as religiously conservative as the majority of America as general principle, more people don't have moral quams about doing it, so more people are willing. Again, we're back to driving prices down due to increased supply.