Cruz was up against Kasich at the end too. And he’s generally well liked. But the point at where he gets subbed in as the candidate instead of Hillary is about the GE.
The rhetoric going into the GE was that republicans should vote for Hillary since even though she is clearly against their ideology Trump was a crazy person and at least she is sane and a reasonable politician.
And I see you’re saying “but Cruz isn’t” but that’s because you’re approaching it as a liberal.
In the minds of the 2016 conservative voter, Hillary was an evil murderer actively put the nation’s security at risk for personal gain.