For one very simple reason: the top line will increase over time to allow for newer higher performance hardware. Vista has a 5.9 scale, and the W7Beta has a 7.9 scale. You can expect the scale to be higher when new generation hardware comes out. That way, older systems will always keep their score (unless hardware is upgraded), ie a Windows Experience Index of say 3.5 will always stay at 3.5, so over time there's a real measure of relative performance over hardware and Windows generations.
If they always had an out-of-10 scale, then everything goes wonky over time and all relative performance meaures disappear because the same hardware will have a different index depending on the version of Windows it runs.
Think about it: what would a "maximum" say 10/10 performance index mean to anyone? There's no such thing and no such system -- it means zip. The MS method of an upward sliding lid actually makes the best sense.
Microsoft actually thought this through quite carefully.
