Someone voted for the guy.
AZ Repubs did in droves in 2016 to keep the majority and avoid a D senator in AZ. Rs outnumber Ds in AZ 55/45, but when it comes to the primary kicking him, or any long term incumbent R out, is practically impossible absent a huge scandal.
This is going on right now in the race for AZs other seat, even without an incumbent. The Establishment has the local media sewn up so there is little criticism or push back on the lies, but vicious lies and attacks on their opponents. They also pour in 10 times what Kelli Ward is spending, 7 million bucks v. $700,000, flooding ttv and radio with lies. They then enlist a ticket splitter (Joe Arpaio) or two. Add in a bunch of endorsements from Establishment hacks already in power for years also and voila, incumbent RINO wins primary and Conservatoves get screwed. Over and over.
This is the Establishment playbook. It’s still effective, but less so since the Internet and social media.