Trump is a private citizen. He is not a sitting senator. The people who know Cruz best are the people who work alongside of him on the job. Are all the other senators RINOs? Is Cruz the only true conservative?
Rubio has endorsements from a number of senators. I despise Rubio and don't want him as President, but he is able to get some of his colleagues to endorse him. Surely Cruz can find at least one senator.