No reason is given as Capcom simply states "We are happy to confirm today that... Dead Rising 2, is set for release across North America on September 28, 2010."
It's yet unclear whether this delay will affect the launch of the Xbox 360-exclusive "downloadable prologue" Case Zero due before release or the horrendous live-action movie Zombrex: Dead Rising Sun, though neither has had an actual release date so far.
The simultaneous PC, Xbox 360 and PlayStation 3 launch is now scheduled for September 28 in North America, or October 1 in Europe according to Eurogamer.
I'm not sure if thats a good thing.
It might be a good thing that they get more time but I doubt it is because they noticed that so late.
I've gotten to the point that I rarely question a developer's decision to push the game back. There are lots of examples of games released too early, and very few of games that were harmed quality-wise by being delayed.