It will run on 64-bit OSes as-is, because they support 32-bit programs. But if you check out Raymond's response to a comment, he explains that at the time, 32-bit apps weren't supported in 64-bit Windows until much later. And even once it was possible, it wasn't considered proper to ship an OS component that wasn't actually 64-bit. (Note that while everyone's still been using 32-bit Internet Explorer for the most part, 64-bit IE has always been included on 64-bit Windows OSes.)

IIRC Windows XP 64-bit was supposed to be a pure 64-bit OS - no compatibility layer for 32-bit apps. Which is most of the reason it didn't get some great adoption.

Later versions of 64-bit Windows had that compatibility layer.