Because people have been trying to build flying cars for a long time, haven't really had any success, and in the process have exposed most of the problems with

Because people have been trying to build flying cars for a long time, haven't really had any success, and in the process have exposed most of the problems with the concept. Since most people understand flying cars and have a pretty good idea as to why they can't be built, it's much more difficult to bamboozle someone out of money by telling him you're going to build a flying car. Things that either haven't been attempted (at least publicly) before and/or rely on physics or other underlying behavior that isn't widely understood have a much better chance of getting funded. It's just easier to snow someone with something they don't understand.