Maybe because that power could later belong to a GOP president? The GOP gave Clinton line-item veto power (that eventually got overturned by SCOTUS).
A line item veto on budget items should be a good thing to conservatives.
Obama could also be reelected. Doing it now is stupid given who is in the WH, i.e., the most radical Leftist ever to occupy the office of President. The last thing we want to give him is more power.