It take's a lot to construct a new universe and characters, even more to pitch that idea to a company, get complete control of its direction AND get people to ultimately buy into that new series.
that's a shit load of work to do when you already have a rich universe that you can still explore. (btw, just in case your thinking it, this is not like the cod or battlefield series because each MGS game has an intricate story along with a bevy of improvements, of which I'm sure hideo spends years tweaking.)