I think Dale B. has given a great summary of the main problems. I don't think thever wrote a Windows version from scratch. In P4 at least, you can find traces of a cross-platform product they apparently used to translate the original Mac code to run under Windows. Such a conversion is never as effecient as a rewrite. As for 64 bit, there are plenty of 32 bit 3D applications that work fine. They could probably cobble something that would run 64 but unless they design it with modern memory management from the ground up, don't expect any miracles.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken