The band went to great lengths for MTM, one of them was choosing a new mixer. LP wanted to expand their horizons and sticking with the same mixer would have pigeon-holed them somewhat. They had Andy Wallace mix the first two albums because he fulfilled their vision at the time. Neal Avron mixed MTM at Paramount Studios.
Andy Wallace mixed the new Atreyu album. I guess it's just that LP wanted to do something totally new this time around, and they felt like they he wouldn't add to the vibe or something. I like the mixing on MTM anyways.