The answer to that is threefold.
Defensive Tackle remains the most critically important position group for the Bearcats this year. While they are developing nicely this camp there was still the possibility that the Bearcats would need to throw a freshman into the fire simply to have a deep enough rotation to keep everyone fresh. Robert Prunty could probably live with that, but its not ideal for anyone, redshirting Ja`Merez Bowen and Cortez Broughton would be the preferable. Moving Harrah inside to defensive tackle makes that a near certainty.
Harrah is not a natural defensive tackle*, but he is big and powerful enough to play as a three technique tackle. What Harrah brings to that spot is pass rushing technique and quickness. Harrah picked up 4 sacks, and 10 TFL's last year. He was a pretty good defensive end at the end of last year. He could terrorize guards as a three technique. The Bearcats will need that from the guy paired next to Brandon Mitchell who is a near lock to play as the run stuffing nose tackle in the Bearcats hybrid 4-3/3-4 defense.
*His natural position is at strongside defensive end
That is the other thing that Harrah has in his favor, scheme versatility. He is going to be the kind of swiss army knife player that the Bearcats haven't had on the defensive end in a while. Harrah's ability to play three of the four defensive line positions gives Prunty and Hank Hughes a lot of options tactically. Remember the 2011 defensive line benefited immensely from Derek Wolfe and John Hughes ability to play multiple spots on the line. One of the most effective tactics for that team was using a three man line with Hughes and Wolfe at the ends and Brandon Mills as a nose tackle. Harrah is a different player, but the freedom he gives Hank Hughes and Robert Prunty to array their forces in new and interesting ways is probably the most important aspect of this change.
The third and final contributing factor to this, Mark Wilson has been really, really good. The redshirt freshman from Hampton, Virginia has been so good in camp that they have to find a way to get him reps. Wilson is still a little light for the position and isn't yet ready to be an every down player. But he can create havoc in pass rushing situations. Plus they need to get Wilson and his younger colleagues relevant experience this year. Before moving Harrah the Bearcats were looking at a situation where they would need to replace three of the top four defensive ends next year. Then there is the possibility of Silverberry Mouhon blowing up this year and declaring early, in which case the Bearcats entire pass rush is gone and they start over from scratch. Moving Harrah free's up valuable reps for Wilson and anyone else who can show that they are ready. Its a win, win, win.