This is all speculation. No one knows exactly why he made the decision he made. From a football perspective, maybe he feels he has a better chance of playing quarterback at WK. Steve McNair had a full ride offer from Florida, but gave it up because Alcorn St. promised that they would keep him at quarterback. I may be wrong, but I think McNair had quite a lengthy NFL career and was the last guy to have his 'competitive fire' questioned.
Carnes stated his reasons.
Carnes was only recruited as a QB, and would likely be ahead of anyone not named Lee as a passer on our squad. That alone would make sure he stays at QB.
McNair was 'recruited' by larger schools, but only MSU and Ole Miss offered, as a DB. McNair preferred QB, and found a place he could play QB.
Carnes situation is not similar to McNair, in any way.