It's California. There's an open primary, I happen to live in one of the few red parts and for one spot the top two vote getters in the primary were Republican.

They didn't run unopposed, the opposition just all failed to get a consensus. CA's system isn't perfect, but realistically a Democrat doesn't have a chance in this particular district, maybe the less evil Republican has a shot though.

US elections involve some compromise. Vote for your ideal candidate in the primary, but after that there's generally only 2 people who have any shot at a given seat, and throwing away your vote in protest helps the worst candidates succeed. That's why elections spend so much money on attack ads - they want you feeling discouraged.