I know some will point out the Jerry Tillery example, but he was certainly more of the exception, rather than the rule.
Offensive linemen are going to generally lack the higher level of quickness that defensive linemen need to succeed. They have the advantage when the ball is snapped, that they can move first with a planned snap count, and move according to a planned play.
Defensive linemen, on the other hand, are trained to react to the ball being snapped. While they also follow a play on defense, their thinking and actions have to be a lot more flexible.
Even offensive linemen who were standouts on the defensive line in high school are going to have issues with adjusting to the speed of the college game if they play on defense. Going against complex zone blocking schemes can easily result in an inexperienced (college wise) defensive lineman getting all but blindsided and taken out of commission by those giant earth movers on the offensive line.
Looking back at Chris Stewart (part of the 2006 class), he volunteered to play defensive tackle, but rarely saw the field, even though we were terribly thin at defensive tackle during the Weis era.
Once he moved back to offensive line, he became a starter, and did a fine job for us, since his skill set and mind set were much more optimal for that position.
I'll be clear about this... It wasn't his IQ that prevented him from being a defensive lineman, since brain smarts and reaction methods are two different things. In fact, Mr. Stewart is now the Vice President and Assistant General Counsel for AIG Investments, and I'm damned proud to call him a fellow alum.