I agree, but linebacker is probably one of the better depth positions. I would have welcomed Carson as a MLB also, but i think the depth may have been an issue, as it is for many recruits. I'm just guessing here, I haven't read anything about it.
In 2007, ND got Brian Smith, who has essentially locked down an interior spot in his 2nd year. In 2008, they got Filer, McDonald and Posluszny, all of whom I think will be MLB or weakside. Filer is already getting reps and will probably take over Mo Crum's spot next year. In 2009, Calbrese is already commited.
PSU despite being "LB-U" doesn't have a LB commit besides Carson this year. Last year they had 4 (2-inside, 2-outside). Year before that i think they had 1-inside.
he also may not have wanted to play FB, which is one position CW wanted to try him at.