It's also a bit of chicken-and-egg thing with normalization in the culture - wherever you go in Japan you can generally find affordable accommodation not too far, whereas in Sweden it's more of a luxury.
Campsites are also great because you can rent all the equipment you'll only need once a year, for the summer camp few days with your family as the Japanese working force does not have a lot of holidays.
Also the equipment can get expensive fast if you want to camp comfortably, and would require storing space the Japanese people do not have at home and thus would need to rent ...
Nevetheless, I think camping is getting popular and more and more people are buying equipment.
And it has indeed been a growing trend in the last couple of years, specifically with heavy imagery and inspiration from Scandinavia.
My pet peeve is getting asked "I [heard/read] that people put chips in their arms there?!" (presumably for payments..?) - both by Swedes about Japan and Japanese about Sweden.
One difference I absolutely do see is what it means to "go camping". Growing up it usually meant bring your gear, hike out, picking some spot in the wilderness. In Japan that's something that is quite rare, "camping" almost always involves campsites.