Because this is the first year Brian Hartline has ever held a title above position coach and if he really wants the Ohio State job down the road, getting some experience, especially in conference, probably isn't a terrible idea. And $4-4.5 million a year is a lot more than $1.6 million. His ability to recruit obviously makes him attractive, but if you're a bigger program, gambling on a 36 year old with very little play-calling duties is a decent risk. IU however couldn't get any worse. Now, it's very possible OSU has named him CIW internally and they won't let him leave, but I imagine Hartline at some point is going to want to take on a role where he has much more responsibility.
I like Shepard but he's like the D-list version of Hartline. Has never held a title past Co-OC (which he didn't call the plays) and I think just lacks the experience to be thrown into a job like IU.