I apologise for the long comment in advance 🙃 I've tried to space it out to make it more readable if anyone is interested in reading it
It could be one of many things, like
a) bot theory as you said
b) people tend to upvote more for the memer than the meme and imgflip has become more of a popularity contest than just a community of memers
c) attention spans are shrinking so if you post a still image or an image with too few colours/transitions/etc will get scrolled past, even if it's a high-quality meme
d) in my opinion, people often subconsciously change what they find funny to fit in, and I mean, who doesn't want to fit in? They see the popular memers' content, realise that that's what everyone likes, and adjust their sense of humour accordingly. They see the small creators' memes and realise that their variety of humour is not popular and won't help them fit in, and they adjust their sense of humour accordingly. But this is just my opinion
In general, I feel all of these factors combined create a system where memers who were already popular get more popular, while the underdogs stay where they are and get overlooked.
But this is all just my opinion, I'd love to discuss this
Caption this Meme