Because Penryn just shipped two days ago?
It usually takes a few months from the time a chipset first rolls off the assembly line, to when computers with those chips are ready to sell. Apple's not going to have computers ready tomorrow based on chips that it doesn't even have on hand yet. Given there's only a month and a half left in the year, and we're in the holiday shopping season, I doubt Apple will want to shake things up and annoy potential buyers until after Christmas, even if it COULD release a Penryn based MBP in time.