Neither candidate will win, but they are the only two who have a rational chance at getting. If they run Cruz and he wins, then things are fine. If he loses, then they can say they ran the most conservative of candidates and lost - which allows them to start pivoting to the middle for 2020, which is what they have wanted to do for years but been unable to do so thanks to the tea party, talk radio, etc. You'll probably see Cruz pick a moderate VP who will then be lined up to be the nominee in 2020.
If Trump wins, then it doesn't allow them to do anything. He'll go down in flames, and then Cruz will get the nomination in 2020. It sets them back far longer if it's Trump as the nominee. At some point, they have to run Goldwater V2 and have the huge loss, and better to get that done now than further down the line.