Cruz is a poor candidate who doesn't have any crossover appeal and is unpopular with the rest of the party for his shenanigans.
Washington politicians aren't Republican voters.
It's not like he can just ignore them and play at being anti establishment. Being the level of unpopular with your colleagues Ted Cruz is is just unsustainable. Cruz isn't very popular with the base either, he's underwater in approval polls right now (
https://texaspolitics.utexas.edu/set/ted-cruz-job-approval-trend-0). However, he isn't running unless he wants Trump to end his career.