Part of that is California's particular political history, where for a long period of time both major parties on the state level were pro-gun-control. The public displays of firearms by the Black Panthers in Oakland in the 1960s scared suburban white conservatives into calling for gun-control laws, which were enacted under Reagan's governorship with bipartisan support in the state legislature [1]. As the state has swung more solidly Democratic in subsequent years, they've retained and gradually strengthened such laws. The state Republican party has now turned towards pro-gun viewpoints, but no longer holds a lot of offices.

[1] The first one, which effectively outlawed both open carry and concealed carry of loaded guns: https://en.wikipedia.org/wiki/Mulford_Act