Simple..
Cruz has honor... Cornyn does NOT..
Cruz protects..... Cornyn does NOT..
Cruz has principles.. Cornyn does NOT..
Cruz is a patriot.. Cornyn is NOT..
Cruz is not a team player, Cornyn is a team player.
The team will back Cornyn and eventually, the team will dispose of Cruz.
Or Cruz wants to be the Senior Senator from Texas. It is not like he endorsed another nominee. He sees the positives for him getting rid of Cornyn.