Playing the numbers game a little, but California is not as universally Blue as it appears. While it is true that it tends to go for Democratic and Liberal policies, California has three of the top ten largest cities in the nation (Los Angeles, San Diego, and San Jose) as well as San Francisco (13th) which is a notoriously liberal city. To an outsider, California is often a deep blue state because these votes typically are able to drown out the more conservative/Republican voters that dominate the rest of the state, so in the presidential and senate races, where it is winner take all, California would appear to be blue.
However, one needs only to look at 2016's Presidential vote break down by county to see that California is much more conservative away from the coast (the major coastal city centers are very distinct).
Now, with the referendum issues, this can be further broken down. Politician based voting relies on taking in many different issues and finding a candidate that best encompasses all of those issues in your mind. A referendum on the other hand asks one particular question and that's where you have more troubles. For example, while you may think marijuana legalization to be more liberal, a growing subset of young conservatives (the Generation Z, the oldest of whom just turned 18 for this election, are anticipated to be a quite conservative generation when compared to the Millennial and Gen X), as well as Libertarian (typically socially liberal, fiscally conservative) do favor legalization because the prevailing belief is you should be free to do what you want (that doesn't mean they personally will partake, just that they don't want the government telling them they cannot). So it's possible that the crossover to legalize is much greater than the crossover to repeal the death penalty.
To turn this around, California liberals aren't universally against the death penalty. While I don't have demographics to support that the same thing happen, California's infamous defeat of Prop 8 on referendum voting (legalization of same sex marriage in the state) was a shock at the time, but we now know that several minority blocks in the Democratic base (in this case, African American and Hispanic voters) prefer social policies that appeal to more religious conservatism than to progressive liberalism. What might explain the voting is that the subset of conservatives that are not pro-death penalty (again, typically Libertarians) was not enough to offset the more religious conservative voters on the left. It might also be a case of nuance playing a key role... there are those who are for the death penalty for such cases as an inmate with life in prison who kills a fellow prisoner (how does that victim's family receive justice?).
A last thing is how is the question phrased? Sometimes, the people writing the ballot will make it a little difficult for uninformed voters to properly answer by obscuring what a vote in favor of the question means. I recall from a referendum on same-sex marriage in Florida that the measure did not pass because the the question read like "Are you not not not not not not in favor?" and many couldn't tell if a yes vote was going to legalize same sex marriage or not.