I think it's just that he has a long history of simply not getting it done. I think he has almost twice as many INTs as TDs whereas Hill has IIRR a fairly decent TD/INT ratio. Hill has about a 20 point advantage in rating and started several games for the Lions and didn't play too poorly when Stafford went down.
I don't know if this will happen and I do like Clemens, but it makes sense to go with Hill instead.