Halo Reach PC über alles!
It is because they don't understand, they have confused the standards of console players to the standards of PC gamers. They thought that PC gamers would gladly shell out $60/month for a service of which all the features were already available to them free of charge (for a long time I might add.) They also made the mistake of thinking that PC gamers wouldn't bother with trying to modify the game further than making custom maps using the items and obstacles that they provided, so the map editor ended up not living up to the standard of Halo 1's editor (which was already low to begin with in comparison to games such as those made by valve.)
But then, another reason lies with the bogus requirement to have Windows Vista installed. And while the OS wasn't necessarily bad (although UAC was annoying at times it could easily be turned off.) the amount of mainstream media criticizing it and the amount of everyday consumers receiving problems with it (mainly due to their machines being underspec) was overwhelming (so much so that M$ decided to wait until systems meeting the requirement were more common before releasing an OS that is almost exactly like it, but that is beside the point.) And considering most casuals at the time ran XP, they couldn't run Halo 2 because they were too afraid to take the risk of buying Vista.
But ultimately it wouldn't have helped them, for the noobifications of H2X were still present. As were the outdated graphics (all they really did was resize the textures and formatted it in widescreen, but to be fair it probably couldn't have been helped considering the size of their development team.) Although I do think Microsoft is starting to gain an understanding of what PC gamers want, considering they stopped charging for GFWL, they still have a long way to go if they ever want to be taken seriously in the PC gaming market.
In conclusion, Microsoft doesn't even want to acknowledge the fact that Halo 2 Vista even existed let alone that it was a mistake, (hence their cutting the support cycle only a year after it came out.) But I can guarantee you that if tomorrow they addressed these issues head on and tried to revive Halo 2, then and only then will there ever be a decent community of people frequently playing H2V.