He ran unopposed because it would cost money to endorse a candidate to run against him and then financially support that candidate in a general election that they'd surely lose to a Democrat. It costs no money to have him run unopposed, not support him in either election and have him lose. The GOP isn't going to waste money trying to gain good public relations with people who would just criticize them for having to run another candidate against a literal Nazi in the first place. It's just bad all around. But one of the bad options is free. Practical solution.
I'm honestly surprised he only got 20k votes. Would have expected more.