Halo 3: Recon is partly a product of Bungie's sorta-kinda split with Microsoft. So says Bungie.
Speaking in an interview, Bungie writer Luke Smith said of the game, "It's also a product of our schism from Microsoft; where now, one of our core tenets as we left Microsoft is to be a multiproject studio that's able to focus on new IP. So as we begin to focus on new IP, and we have multiple projects spin up at once, [the idea of] meeting our fan desire for more Halo 3 content, and doing it with a team of incredibly grizzled veterans at Bungie, made perfect sense to us."
Translation: if Microsoft were still the boss of Bungie, it would have the whole studio plugging away at one new game and it wouldn't be an expansion.