I'm trying to recall right now, and maybe the only places where I didn't find salmon in any of my visits was in the more expensive/ fine dinning places (think ¥8000+ places). All the common kaiten sushi places, or chains like SushiZanmai have all kinds of salmon pieces. Different levels of fat, broiled, rolls...

Is it maybe a bit more like "comfort food", for kids, for less delicate palates, cheaper? Maybe. But truth is I haven't asked why I haven't seen it in the expensive places.