You know, that's the thing with people - you never really know what they're going to respond to. We all have slightly different senses of humor, and at least some of the time, it's something other than the humor that resonates with people. Popularity of a meme is also a positive feedback loop. So, the more upvotes and comments a meme gets, the more exposure it gets, and so the more upvotes and comments it gets, and so on (until the point where most all of the regular/daily users have seen it). Some memes are really good, but for whatever reason, never hit the tipping point that propels them to the front page.