McCain was the establishment candidate and was selling himself on the basis that you had to have a “maverick” to attract the mushy middle. It is the same argument over and over again, and it works with a lot of conservatives who lack discernment.
The funny thing is that his RINO-ness was still largely supported, quite reluctantly, by the conservatives. While the Cornhole Powells of the world, who insisted it takes a RINO to bring the party into the mainstream, bolted for the exists, leaving us holding the turd bucket.