As the chart suggests, California Democrats are much further left than national Democrats, although California Republicans look pretty much like national Republicans. Also, Abramowitz notes, liberals and conservatives are geographically concentrated in a few key areas. If San Francisco liberals are so far left, and there are pretty much no conservatives in the area, then legislative districts in that area will keep electing far-left liberals.
It's interesting to note the slight discontinuity between Abramowitz's voter graph above and Boris Shor's graph of elected officials. In the latter, California's Republican legislators constitute the most conservative legislative party in the country. And in general, it's worth remembering that elected officials tend to be more ideologically extreme than their districts. So the graph above only tells a part of the story about legislative polarization.
(h/t Brendan Nyhan)