basically Kojima saying Hayter is not capable of doing facial capture while Otsuka Akio can
This actually may be a big part of the decision.
We all know Kojima is a Hollywood fanatic. Japan's live-action scene is.... eh, and Akio Otsuka is already a high caliber seiyuu, so there's no real incentive to replace him either (and it is reasonable to guess that Kiefer's motion capture is going to be driving Big Boss, not the Japanese side of things).
But you have David Hayter, who despite being Snake for over a decade, really isn't that much of an actor. And no, Hayter's performance in Guyver 2 doesn't count. Yeah, he's a good script writer (and a debatable voice actor), but it seemed like Kojima really wanted to drive both a Hollywood focus for MGS5 and good motion capture. And if a good actor with a voice that can somewhat fall in line with Big Boss is actually interested, I could see Kojima jumping on that chance in a heartbeat. So now Kojima has a good actor that can really drive all of the new FOX Engine mo-cap shit better than Hayter or some other person acting independent of the voice could have, and potentially a good voice actor.
This is my speculation, but on the other side of the coin, Sutherland might be expensive as hell if he's doing both facial mo-cap (and given his Jack Bauer experience, maybe mo-cap in other areas too) on top of voice acting, so we'll see what Kojima does with this after MGS5. It either could be the beginning of a new relationship, or it could be a one-off thing.
Yeah, it sucks that Hayter got the shaft in the way that he did. But I can at least understand why Kojima jumped at this kind of opportunity. Plus, it isn't like Hayter didn't have a good decade-long run.
Edit: C'mon Hayter. That post would have actually been classy, but that New Coke dig wasn't necessary.