Part of it is that comics have lead a sort of underground, partly-ignored existence. They began life as something used to sell newspapers (editorial cartoons, Sunday-funnies, etc., etc.) and have been fighting to get rid of that brand ever since - not that there's anything wrong with doing gag-a-day strips, or comics for children! Those are both great! But there's a pervasive attitude of dismissing things that are funny as non-serious (best example I can think of off the top of my head; people dismiss Calvin and Hobbes as a gag-a-day strip, when Watterson used it to discuss things like the meaningless of existence, the corruption of political systems and the wonder of the universe. It's just that he managed to be funny at the same time), and that definitely dismisses media aimed at children as something not worthy of respect.
Which is, erm, entirely stupid. The media we consume as children help shape our perception of the world, so it kind of really matters. But this lack of respect means that comics aren't given the same attention as other mediums. Imagine if comics were discussed and reviewed and cared about by the newspapers and tv-news the same way, say, movies are; we'd get a LOT more mainstream acceptance, because we'd be normal. We'd be like the rest of them; everywhere.
And when you're everywhere, people care about you because they kind of have to.
There's also the problem that comics, to an unfortunate extent, are divided into two target demographics - small children and adults. There's very little in between. So you can start reading comics as a child, but eventually you hit a point where there are no comics aimed at your age-group - and you're too young to start reading the comics aimed at adults*. So what do you do? Well, you drift away and stop reading comics, because they're not for you any more anyway - and so the reader-base for adult-aimed comics shrinks. You only get the people who managed to stick around - or who, by some magic, decided to start reading comics as adults. If there was a bridge between children's comics and comics for adult readers, we wouldn't see that same kind of shrinkage.
And then there's the fact that reading comics is like reading a new language. You're not just reading the words - you're reading the pictures, and you're reading them at the same time. In a well-crafted comic, things like the size of panels matter, because they indicate the rhythm of the story. Small panels are read faster - big panels are slower, because you stop to take in the scenery. The reader has to follow the path of the word balloons/speech bubbles across the page, and at the same time keep track of the visuals without getting lost, AND they have to get used to "hearing" a comic; that is, they have to process visual representations of sound - and it's hard. I've been reading comics since I was old enough to read, and my mom read Asterix to me before I could read them myself, so you could say I speak comics as a native language. A lot of other people don't - so there's that threshold to get over.
Also, comics-readership varies from country to country - the same way comics-creation does. The Japanese comics-market is a lot bigger, percentage-wise, than it is in almost any other country. The American comics-market is - to my Swedish eyes - weirdly divided into "mainstream" (superhero-comics, with their 1930s-40s pulp origins) and "indie" (literally everything else). The Swedish comics-market is tiny, and the ones that "make it" (ie: the ones who can make a living off of it) are either gag-strips or autobiographical comics - that is, if it's original content. A LOT of the Swedish comics-market consists of people working on one of the long-running franchises (Bamse, Kronblom, 91:an Karlsson - or The Phantom, an old American pulp-hero that the Swedes have pretty much adopted as their own).
In short - comics are an artform that produces ASTONISHING works, but they just don't get the attention they deserve until they're translated into another medium, like movies.
*) This is not a reflection on what individual people are capable of reading; I'm just saying that there's a big jump in content and tone between, say, Asterix and Watchmen.
@getsuart Wait, wait - you're French, so... she declared that at Angoulême? Seriously? D: Goodness me.