I have no support for this theory whatsoever, but I wonder if the problem with Bone had to do with, basically, just the fact that the main characters are little blobby cartoon-looking guys?

(This is not a value judgment I love Bone with great intensity don't kill me)

None of the other comics you listed are very "cartoony," and given the sort of places a kid in the US right now would usually be seeing that style of art - that is, mediocre-to-insulting newspaper comics and cartoons - I think it's sort of reasonable that they would have trouble getting into it. They might never have run into anything good in that particular aesthetic before. If a reader doesn't trust the medium, she won't risk the kind of emotional investment she needs to get anything out of a story with an epic-type structure (if "epic-type structure" is not a completely obnoxious phrase). Therefore, the story becomes boring.

I suspect that this is why I still haven't managed to finish Lord of the Rings. I read so much really bad epic fantasy in high school that I now mistrust it and keep it at a distance, which isn't a good way to read.