The problem is there is no easy way to distribute new software to Android devices. Heck, just getting an updated Android is a crapshoot unless you have a very popular device. Ask the cyanogen mod guys.
With a PC we have a open and understood boot process and fairly standardized hardware. Removing the OEM preloaded OS and installing a free OS on a PC is therefore fairly easy. Each Android device has to be undertaken as a new port. Without even a plan to crack that that problem there isn't much point trying to make a tablet or phone version of any free OS.