Why was Dawn canceled? The answer is perhaps not all that clear, according to Mark Sykes, a co-investigator on the axed mission. Yesterday's Congressional hearings stressed that smaller missions should not be sacrificed for bigger ones, yet reading Sykes account, this appears to be what has happened with Dawn, and only hours after the hearings ended. This story is interesting, and no doubt will continue to unfold.
Update #2: According to NASAWatch.com, Dawn needs $40 million to complete, but will cost $10 million to kill. So, for a $370 million dollar mission, it's getting canceled because of less than 10% of its total cost.