Akro,
I don't know if I believe it myself but I read that it has to do with the high mountains (compared to most areas in Europne) in the US. Altitude until well above all that cumulogranitus, FLs after that.
Personally, I find what Nick is saying more believable.
Hmm, I can imagine that sectors (in this case significant as areas with the same QNH) tend to be larger in the US as well, making HOVs and flying on altitude less of a PITA?
Any other suggestions?
Cheers,
/ft