I think Reddit's voting system, in combination with its popularity, is what selects for humor. People, and especially laymen, are much more inclined to upvote funny posts that make a big impression on them, than they are to even read insightful posts fully. That's why the top comment is always a joke, and the top subcomment of that a follow-up joke, etc. This is especially the case with popular subreddits and the front page. And the widespread abuse of the voting system also discourages discussion: people downvote you for saying something they disagree with.

So unlike others, I don't think it inherently pertains to the type of person who visits Reddit. Although that also contributes, I think any community that becomes big enough will struggle with this type of problem. Just look at 4chan. There used to be various boards where you could have in-depth discussion with intelligent people. But now the discussion is drowned out by memes or single line comments, and the intelligent people long driven away.

I don't think Saidit's alternative voting system does much to address the issue either. People are already abusing it, voting insightful on a humorous comment. It's mostly its small size that keeps it this way.