Eridani74 said:
As a lot of things in the industry it comes down to "we didn't bother, we thought it was too expensive/complicated/time-consuming/etc".
...But as a gamer, I'm duty-bound to demand things even if they're wildly impractical or only apply to an audience of me!
More seriously... Yes, I know this is true, and largely recognized it even before I made the original comment. I don't think it's impractical to keep a single Vista machine running in the back somewhere, but the maze of 32-bit and 64-bit versions, different drivers (and companies discontinuing new driver releases with particular OSs), background programs like virus-checkers and disk emulators... makes it far more complicated than that.
I sort of wonder if there's a niche to be filled here for a small, third-party company that just keeps a small lab of slightly-obsolete computers running for this purpose, but I really don't know if there would be enough of a market to keep such a company afloat.
So... grouse, grouse, fume fume.
Edit/Add: I have to confess, though, that I find it annoying how difficult it often is to even get second-hand information about this kind of thing from the player-base. Just one person saying "I'm running Vista, and I can play this just fine" would take off a lot of unease, but one can sort through hundreds of pages of Steam comments and Google pages trying to find one such comment.