My guess is they each got pretty low draft grades, indicating they’d be picked in the later rounds–if at all. Each of them probably saw what Lloyd did for his draft stock this last season and figured they could significantly boost their stock by coming back a year as opposed to trying to improve their position by doing a few drills at the combine.
Just saw this story from the Trib on the situation, and it sounds like the College Advisory Committee did not give Kuithe or Kincaid 1st or 2nd round grades, meaning it was recommended they return to school.
Fred Whittingham said this years NFL draft is LOADED at TE and next year will be a more regular year for TE’s entering the draft. So it was a timing issue on that front.