I do like the bike racks on buses quite a bit as a concept... but as another poster mentioned, I never expect that I will be able to put my bike on one of them.
Just a wild guess, but I'd say that the distances are usually shorter in Europe (because of increased density), thus bi-modal commuting is much less of a necessity or of a demand made by commuters. Also, in a lot of cities where lanes are extremely narrow, I think it would be quite a bit more challenging to drive buses with a rack on!

