The anti-grav belt is the purest object of individual freedom. But it is a wild freedom that we have less of a frame of reference for.
One thing for the flying car's appeal is the relatability of it. Everyone has been in a car, and most people have driven/purchased/dreamed of some car or another at one point in their life. Even for folks who are not auto nerds, there's some marketer or an auto nerd in your life that has instilled or shared some piece of the dream in you. In addition, I think the flying car carries more appeal because it is freedom that wraps the passenger with the sense of security that cars provide. Auto manufacturers have been working as engineers and marketers to firmly associate safety and cars for many decades. Granted, installing safety features in flying cars would be more challenging as crumple zones and air bags won't cut it.
With regards to steampunk, we can't discount the overall cultural changes that is has brought along as well. I can't count the number of pure steampunk costumes at cons, and then you heap all the fusion steampunk costumes on top of those. Anything popular enough will receive bandwagon jumpers (writers and fans) that will degrade the perception (or quality) of the sub-genre. I have also noticed that all of the 80's and 90's goths that didn't want to assimilate into suburban life have jumped into steampunk with both feet since emo ruined the whole dressing in black thing, and goths already had a bunch of Victorian in their closet. I imagine that's had an impact on the perception. The mainstream will have trouble accepting steampunk if the best mass-market example they have is a Will Smith summer blockbuster.
From a fandom perspective, probably the best thing for steampunk has been Firefly/Serenity. Still not mass acceptance, but definitely a better presentation.