The 21st is the only targeted district where the Republicans saw a gain instead of losses. (What's with those people, besides widespread poverty?)
On searching, I found this WaPo partial explanation back on June 7. Possibly ceding the 21st to the GOP was a very deliberate, strategic decision.
The DCCC’s long national nightmare is over.
The Democrats’ House campaign arm, which had become a punching bag for conservatives and liberal insurgents alike, had one job Tuesday: avoid a lockout in California. It pulled that off, with four strategic decisions.
First:
It encouraged T.J. Cox, who lived in the 10th District, to run instead in the 21st, a play that probably prevented a lockout in what had been a crowded primary.
Second: It endorsed Gil Cisneros in the 39th District and spent money to bring down the numbers of every Republican except Young Kim, who had surged to first place. Third: It endorsed Harley Rouda in the 48th District and spent $1 million to attack Scott Baugh, a GOP leader in the district who finally conceded the race Wednesday afternoon. Fourth: It attacked every Republican in the 49th District except Diane Harkey.
This has all been widely reported. What surprised Democrats was how little Republicans intervened to screw it up.
Democrats expected higher-than-usual turnout but feared, correctly, that voters excited by the midterms had little idea whom to vote for. Indeed, thousands of voters in the 49th District backed candidates who had already quit the race, while ... They were also confident that at least one Republican would make the runoff in each race ...
https://www.washingtonpost.com/news/powerpost/wp/2018/06/07/slow-waves-high-turnout-and-no-lockouts-four-lessons-from-this-weeks-primaries/?utm_term=.753828846547
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