I’m a bit curious too. My theory is that it may have come down to licensing and trademark issue. Since the game used actual car brands and their logos and such, they may have had some agreement over their usage and perhaps the period of that agreement ended. It’s worth noting that the game was available from several different stores, and it became unavailable everywhere as far as I know.
I’m pretty sure there were flying mounts even before Skyrim. Hell, go all the way back to joust.