I must admit I too feel as tempted as always to go "dense Americans and their superhero comics", but here the reason is probably simply that a big part of the humour is done by poking fun of European national stereotypes, and also regional stereotypes of France. None of that translates really well to an American audience.
Asterix is incredibly dense with jokes; you can miss lots and still find it hilarious.
when i was a kid many school libraries (do schools still have libraries?) had all the tintin and asterix books but no other comics.
I probably missed a lot of the jokes. That’s fine. I got a bunch of them, and I laughed at the goofy drawings.
It's read all over Europe (and other places, including Canada and Latin America), in countries who are not familiar with the "regional stereotypes of France" (nor do they have to be to appreciate it).
As for the references to "European national stereotypes", I don't see why they should pose a pose to Americans (if them being dense wasn't the problem). They are quite basic (doesn't require any intimate familiarity with the countries besides a basic knowledge), so if they weren't mostly ignorant when it comes to foreign cultures, they would have recognized those too.
Yes, there are jokes that get missed. But so many are understood that it was still funny.
America had its own influential comic book industry since the early twentieth century, so it never got into importing them. For most of Europe (and beyond) the comics made in France and Belgium and translated in the vernacular were the absolute cornerstone of comics for most of the previous century.
A lot of European kids who grew up in the twentieth century know about the American Old West through reading a cowboy comic (Lucky Luke) made by a Belgian, and first heard of Julius Caesar and the Roman Empire through Asterix.