The reason is that it forces the pro-amnesty wing of the GOP (people like Rubio, McCain, etc) to oppose amnesty, because they don't want to be seen as supporting Obama and his executive order.
Had George W. Bush (another amnesty supporter) issued the same executive order, it would have stuck. Democrats would have supported it because it was amnesty for illegals, Republicans would have mostly supported it because it was Bush. But since it's an Obama executive order, even the most liberal GOPer's don't want to risk looking like they're on his side. Therefore, by early 2015, the new GOP House and Senate will find a way to overturn it (albeit mostly out of partisan politics rather than principle).
What is the Executive order?
It was posted that it is two memos establishing study groups at agencies