And of course for cost reasons. The PS3 was already a cost bloated console. Moving up a generation to equal or better the Xenos would have meant more costs, read losses, for Sony.
Of course, cost is a major consideration.
But still Cell + RSX >>> Xenos + Xenon and that's what matters as well. In addition to this default harddrive, Blu-Ray capacity and other extras enlarges the technical advantage and yes those extras results into extra costs. But the PS3 with all that is truly an amazing deal at $299.
Uncharted 2 will certainly drop jaws, also people with mega expensive PCs.