In comparison to the U.S.A., there seem to be far less leading or successful game companies in Europe.
The recent salary survey of GameCareerGuide shows that the average salary of a games programmer in Europe is only 46k versus 80k in the U.S.A. While only 378 European developers replied to the survey, 1014 developers from the States provided usable feedback, which might also indicate that there are less people employed in game development in Europe.
What makes it so much harder to develop games professionally, both AAA, indie and casual in European countries?
Note: the total number of game companies in both continents seems to be rather equal (see wikipedia for a complete list, around 117 companies are U.S based versus 106 European). But we have to take into account that, although North America is 2.5 times larger than Europe, there are many more people living in Europe than in the States (731,000,000 versus 309,944,000).