I think that the Olympia planners are trying to compete with the success that is the Arnold Schwarzenegger Classic. By keeping the Olympia in a single site, they can plan much easier and can add more events and competitions every year. Plus, the majority of the audience is in the U.S., so they are catering to the bucks. I haven't attended either the Olympia or the Arnold, but from what I have heard, the Arnold puts the Olympia to shame in terms of size, production values, etc.
I agree with others, however, and think that they should move the Olympia around every year. Gives it a much better, unique international feel.
How about having the 2008 Olympia in Germany somewhere, so Dennis Wolf's hometown fans can see him win the Olympia!
