The proposal noticeably points out that helicopters are "limited in flight speed and availability." Combat helicopters don't move like combat jets, but it's hard to imagine a flying car that's going to move any faster. One could also point out that if availability of helicopters is an issue, perhaps we shouldn't be throwing money after vertically-launched flying cars. But what's good for the military often ends up being good for civilians as military technology bleeds into commercial use, so we're the last ones who will criticize DARPA for this one.