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.

It's hard to say, depending on how far back your idea of "a few years ago" might be or where exactly you ate. After all, Japan has a lot of sushi restaurants and even different sushi styles. For example, you definitely wouldn't find salmon in an edomae sushi restaurant.

For reference, salmon was the most popular choice in conveyor belt sushi places 3 years ago. https://www.nippon.com/en/japan-data/h01627/

I remember asking a Japanese chef in Japan, in 2019, and the answer I got was that sushi is much less popular in Japan than believed outside of Japan, and a lot more people eat rice and chicken, pork, etc., than salmon.