Money and process. They have to be certified by MS and Sony, maybe Steam. And with Bungie, at least until recently, wanting to remove content, it would be more of a hassle than creating their own in-game system. Fro example, Prestige Levi had to be replaced with a GM -- which is a joke. So rather than having to keep track of that kind of thing, my guess is they just decided to forgo it entirely.