hardware compatibility. For software, this has been discussed to DEATH on these forums. Search for it. Hardware? Try to plug in a BU353, or a Fusion Brain, or a USB to serial converter to control a HD radio, or a Phidgets, or a Boomzbox, or an obd2 scantool, should I go on? Who actually uses a keyboard or mouse in the car? So yes, software has a LOT to do with the tablet form factor when you're talking about Android tablets.
So say you spend the extra $300 to get yourself an X86 tablet (Or some miracle happens and someone actually writes some good software and drivers for Android tablets), thus solving both the hardware and software issues. Without doing EXTENSIVE fiberglassing and modifications to the vehicle itself, there was no way to make it look anything but a cheap add-on. (see: theft-bait). Plus you still have to run the wires just like a carpc, except with a tablet they're all there for you to see no matter how nicely you tuck them away. I've done it, it's not good, and not a single one of the tablet installs I've seen either on these forums, or on the internet, look even half decent, unless they fully fiberglassed it in... in which case, for that effort just install a carpc.
So yes, you can put a tablet in your car, and I'll laugh with it gets stolen, or while you're beating your head against the wall waiting for software. I'll stick with the OEM style look of a full on carpc. It's just a shame the screens aren't quite up to par yet.