Tidal action probably also plays a part. San Francisco Harbor is on the backside (more or less) of the small mouth of a large bay. I’m pretty sure that tidal currents dredge the bottom there so that it’s naturally deep enough for cargo ships but shallow enough to sink pilings and build docks.
The Port of Los Angeles also got an ENORMOUS amount of support from the Federal Government to get all the work done on it.
But to go in to all the reasons about why Los Angeles is bigger than San Francisco or San Diego requires a full length book.
I actually wasn’t taking into account barrier islands. I mean I think Naples Fl is the only city in FL truly on the coast. Portland and Seattle are really away from the coast and Vancouver Island is a REAL island.
I think LA developed because of Hollywood and WWII.
Seattle was originally built on a tidal flat: it flooded partway regularly. Like Vancouver, it was settled for its location (the only damned place you could get to the water) anbd became big due to lumber. After the huge fire, caused by an inexperienced tanner’s apprentice faced with a glue fire, they knocked down the cliffs, ran then down to the sea, and built their city on the incline.
Humboldt Bay is pretty much the only “developable” coastal area of California north of SF. There isn’t much of an expansion economy up there, just a barely sustaining combination of timber production, fishing, and tourism. The rest of the NorCal coast is quite inhospitable to anyone who doesn’t want to chop a lot of firewood, periodically live without some utilities (or off the grid completely), use 4 wheel drive by default, and get good and dirty doing just about everything.
My other tongue-in-cheek answer to the OP is: Because there is a just and fair deity, and she knew that I would thoroughly enjoy growing up in a rural and isolated environment 