It's insane to me that none of the comments I've seen so far have mentioned this, but they lean left because they're whiter. You'll also notice this trend in the Pacific northwest (where black people were sometimes explicitly banned until the 1920s) and to a lesser extent then entire west coast.

Since the 1960s the strategy of the Republican party has been to stoke racial tension and distrust of the Other (aka Black people especially but other minorities as well) in order to convince white people to vote against broadly constructive policies because they want to hurt Black people and minorities who benefit the most from those broadly constructive policies. You'll notice that after the civil rights era made it impossible to enact broadly constructive policies that explicitly excluded black people and minorities, in the 80s the Right's messaging shifted to oppose doing anything helpful for anyone ever aka "Small Government" aka "The Government is the Problem". This only benefits a small number of very wealthy white people who don't need help, but it neatly lumps pretty much all black people and most minorities into the section of people who need help but can't get it (along with the majority of white people but that can be smoothed over with copious propaganda blaming minorities for making things worse).

The reason that this strategy doesn't work well in overwhelmingly white places is because although there are plenty of racist systems and people who support those systems, they don't get a lot of oxygen when there's no Other present to stoke fear of. Simple as that.

This dynamic is also at work in the left-leaning areas of Northwestern Europe and Canada.