Because Apple/Steve Jobs dont put as much effort into their mac lineup? Just look at the Mac Pros, theyre outdated and the imacs run circles around them. Clearly Apple doesnt care much.
The Mac Pros are roughly 1 year old and coming up on the end of their current cycle. Updates will be happening soon.
The only iMac that beats any '09 Mac Pro is the 27" i7 2.8 GHz, which just edges a base model 2.66 GHz quad core MP.
All others will run circles around ALL iMacs.