I don't think anyone knows for sure. It's their site, and their primary goal is making it seem as attractive as possible to advertisers. They can feature whatever they want for whatever reason.
I guess if a meme breaks submission guidelines (the guidelines that come up right before you click submit), or the site's terms (you can see those by clicking where it says "Terms" at the bottom of the screen), it may be less likely to feature.
I think if a meme's sexual references are too strong (remember this site is for 13-year-olds and up), or has too much gore, or deals with a taboo topic, it may not feature.
I've also seen a mod say that they're more likely to pull a meme if they think it will lead to a lot of fights or arguments on the site for no good reason.
Also, apparently if a meme gets a much higher proportion of downvotes than upvotes from other users in the early stages of it being featured, it can automatically go back to submission.
Those are some of the reasons I can think of. It can be a bit hard to tell, because I think there are a few different mods, so depending on which one is doing the reviewing at the time you submit an iffy meme, you may find different results.
That's pretty much all I know about it. Ultimately we're all just guessing based on whatever observations we can make. :-)
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