I remember back when Halo 3 and 4 were out for one Microsoft gaming platform (Xbox), but not the other (Windows); and Halo 2 for Windows was a disaster area (Games for Windows Live, ugh). Went on like that for years and years. I just couldn't figure out why even first party Microsoft games were not available on both Microsoft platforms.
I would have bought the Halos in a heartbeat from a Windows-locked store back then. Though, I'm glad that by the time they launched an official MS store (and much, much later released Halo 3+ for the PC, and Halo 2 in a way that didn't suck), I was already souring on Microsoft as a company and didn't want to lock myself into a platform-specific store.