It wasn't "they", it was Corker. He snuck it into a must-pass bill and there wasn't enough opposition to stop him iirc. He wanted to buy Congress time to draft a bill that actually would kill the GSEs without the administration pulling the rug out from under him.
jumpstart was enacted because they had a deal with buffet where he was going to buy the senior prefered and release fannie and corker put the cork in it at the last minute, you'll see they will finally do it on jan2 2018