For the same reason Windows did not become a mainstream OS for tablets and phones. There is a much greater distance between a finger-touch UX and a mouse UX than most people imagine.

There is also probably a hardware dimension to the answer: There is a greater distance between a heavy "convertible" part-time touch laptop/tablet-ish device and a real tablet that can be carried like a sheaf of paper than most people appreciate.

Because we have lot of Linux distros lot better than Android. Debian, Arch, Fedora, Ubuntu, etc. BTW, there's is a Linux distro called Android x86 for PCs, based on Android.