For the same reason it is a hard thing for Microsoft to run 16bit apps alongside 64bit apps even though Wine shows that it is perfectly possible for the vast majority of them (those that could run under 16bit protected mode Windows 3.x, which is pretty much all of them): it isn't really a matter of it being hard, it is more a matter of them not wanting to bother (or "allocate the resources to do so" in corporatespeak).