I like this little article:
http://www.strategypage.com/htmw/htmurph/20090721.aspx
And this quote in particular:
The problem with the F-22 is that it is way ahead in performance, and cost. The argument against the F-22 is that it provides more performance than the air force can afford. Now, when it comes to performance, fighter pilots feel "too much ain't enough." But to the air force commanders who must plan and conduct the battles and campaigns, too much performance in too few aircraft can be a losing proposition.
It sums it up in a nutshell - the F-22 is almost too much plane for even the U.S. A single F-22 may be very capable, but at the end of the day, the U.S. air force has a lot of airspace in a lot of places that they need to cover and project power over.