hardware support. With the newer releases of Debian, the sub-distros (ubuntu and the like), and others the gap isn't nearly as bad as it used to be, but that rap is hard to shake. Combine that with most "geekish" users at least trying it once only to find their wireless card or something similar in nature isn't installed, isn't supported, research for an hour why/how to fix it, then spend 2 hours on actually fiddling with it to get it to almost integrate properly so it does work with the GUI tools, and that keeps linux in the realm of "close, but hell if I'm running that as my main OS".
I like debian, especially current test releases as they support far more by default in the kernel, but realistically there are some seriously oddball devices most of us are trying to get working for a CarPC, many of which will either: (a)not function at all, (b)won't have as much functionality as it would with the defacto windows support and driver, or (c) have to wait for someone else to be in the same boat and write something comparable for what you can now already get in a windows FE.
I wanted linux for my base when I started, and I'll probably bite the bullet and at least TRY this, and probably a full on debian release with just the FE compiled into it. But realistically, I doubt it will be kept installed as I know from experience something fairly basic that I want simply won't work as well as it did in windows even with research and tweaking.
It's got the same catch-22 issue it's always had, it doesn't have as much as easily because not enough people use it, and people don't use it because it doesn't have as much as easily.