There was a significant drop-off in votes from the Presidential to the Senate race in California. As the linked chart shows, there were 13.7% less votes counted in California's senate race than the Presidential race. In other states the drop-off ranged from 0-6%. The average drop-off was 1.8%. So roughly 12% of California voters who turned out chose not to vote in the Senate race between two Democrats. Many of those were likely Republicans or right leaning independents. So probably more than 40% of Republican voters chose to sit this one out. That has to be a disappointment to those who authored top two. Their thinking was that Republicans would vote for the preferred Democrat in such a scenario.
Some people thought Sanchez could win by getting an overwhelming percentage of the Republican vote. The Senate race wasn't close, however. Even if Loretta Sanchez had gotten all 1.9 million people who voted in the Presidential race, but didn't vote in this one, she would've still lost by over 900k votes. In a statewide Democrat on Democrat race, you can't win by trying to win Republican votes. A Democrat needs a significant share of Democratic votes.
This chart shows the Presidential and Congressional vote in each district. You may see these numbers elsewhere with the percentages of each race, but here you can see the raw vote totals. I separated the districts into those where both a Republican and Democrat ran and those where no Republican ran.
You can see here how dramatic the difference between Trump and congressional Republicans was, especially in Orange County. Mimi Walters got 37,905 voters who didn't vote for Donald Trump and her opponent was unable to retain 33,218 voters who voted for Clinton. Note that I'm not saying Walters got 37,905 Clinton voters. Some of her votes likely came from the 21,982 voters who voted for someone other than Clinton or Trump for President. I say some because while the independent voters might've broken in favor of Walters, she probably didn't get more than 70% of them.
Every race had less voters who voted for congress than President. It's likely that the some Clinton and some Trump voters didn't vote in the congressional race. That's fairly standard nationwide. So Walters didn't get all 144,713 Trump voters. Off the cuff I'd guess Walters got 127k Trump voters, 14k independent voters, and another 41k Clinton voters. Is that sustainable for her to keep winning? I think it is. Mitt Romney got 169,489 votes and a bunch of the Clinton/Walters voters likely voted for Romney in 2012 and voted Republican down ballot this time.