I don't think it's that Halo is a less creative project than Metal Gear, his opinion just probably wasn't taken into consideration and heard working at 343 as it was at Kojima Productions. KojiPro has a huge fanbase in the West, and they had a goal in mind of appealing to Westerners specifically with MGS4, and he was the American guy who could give them constant feedback on that. With Halo, there's no need for that kind of thing, and if his ideas don't mesh with what his peers and higher up people think, he'll be overruled pretty quickly. You could probably just sum it up with "Halo is much bigger than him."
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