Thought the point of external motion controllers was to unload some of the load though... So newer faster probably not a vaild reason.
Yes, it unloads some of the load. However, many of these people wanting to use XP are still wanting to use the parallel port, so the motion controller argument doesn't work there.
Also, consider that virtually all modern software uses a lot more resources than software written 10+ years ago.
Microsoft Office 2007 will run pretty well on a 1Ghz XP machine, but try running the current version of office on it.
Mach3 was written about 12 years ago, and Mach4, even with a motion controller, runs best with a lot more horsepower behind it. Unfortunately, there's no way around that.