Well they chose it for the newer BTX formfactor standard that hasn't gained a great deal of momentum.
I suppose for those other than overclockers, hospitable cooler temps for the psu are more desireable. Since the cpu is directly below the psu and hot air rises and alot is sucked through the internal components of the psu even with the rear case fan sucking a great deal of the hot air out.
::edit::Here's the reason.
Power Supply at the Bottom? why?