Yeah, it's a bit of a mystery. Remember, though, that the superheroes dominating the market is an American phenomenon. While the rest of the world might be buying superhero comics and reading them to a large extent, there are NO homegrown superhero-comics in Sweden that I can think of - unless one counts Johan Wanloo's 80s-action-movies-tribute Rock Manlyfist, and it's just not the same thing.
There are several reasons why comics are finding it hard to break into the mainstream. To read a comic and be able to enjoy it fully, you have to be comics-literate. Unlike prose - which we learn to read early on, are expected to read, and which is so ever-present in most people's lives that the vast majority acquire the skill - comics require a different reading skill-set. I have known many, many non-comics-literate people who get confused about reading-orders on panels and speech bubbles, and who are unused to switching between reading text and "reading" images - which is what one does, with comics.
Learning to read comics "fluently" is like picking up another language - it may be similar to what you're used to, but it's not quite the same, and people find it tricky to acquire unless exposed to comics regularly. And most people aren't. The fact that Raina Teigelmeier's books (and many other younger-reader comics) are such massive bestsellers now are a great sign of progress - but the main demographic for those books have a few years before they're grown up enough to be buying their own comics.
And then there's the issue that we have here in Sweden, at least, and which I think might be a thing elsewhere as well; you have comics for kids, and comics for adults - and very little in-between. Once kids age out of reading kids-comics, they're left with a publishing landscape that offers them very little aimed at them, and that's where we lose a lot of comics-literate readers. They naturally wander off to read something else, like a prose-book, or maybe watch a movie, or play a videogame. There's a gap there, and it needs to be filled in.
Also, unlike novels and other prose, comics do not have the same kind of social prestige. If you tell people you read books, you're perceived as cultured and educated and smart. If you tell people you read comics, you're perceived as someone who is kind of weird and probably childish. While it's been getting better lately - the massive success of superhero-movies has helped, I think - comics have still to get rid of that label of being something that is for kids, or weird people. The stereotype of the smelly comic-book nerd is firmly established in the consciousness of a lot of people, and we have a ways to go before we're rid of it - if we'll ever be.
As if that wasn't enough, I can tell you that one of the main reasons why I don't read that many superhero-comics (I do read some - like Ms Marvel, and any Doctor Strange I can get my grubby hands on) is that I just don't know where to start. There is an established comics-canon over 70 years old, and picking up any superhero book will drop you into a universe with a million different plotlines running in a million different directions, and even if they just feature in the background of whatever comicbook you happen to be reading, that's still a threshold to get over. If you can't learn to let go of that, and be okay with being confused about some things that won't be explained unless you read a dozen other comics as well, then getting into the genre is hard.
A good writer can keep you hooked anyway, even if you get no explanations for stuff - but a lot of people like reading self-contained things, and that's okay too. It just means the superhero genre doesn't appeal as much to them - and as that's the established mainstream in America, it means it's got a narrower appeal than it could have.
