It's simple and it's easy to teach others. All the characters are very simple shapes - circular heads, generally rectangular torsos, simple faces, and since EC in particular uses that Rayman-esque style of floating hands/feet/heads that's even more lazy and ugly time-efficient. It's easy to keep on-model because the model is so simple and because of that it's easy to cut and paste - every single humanoid on the channel is basically a recolor just with different hair/clothes. I believe that EC uses multiple artists/has had different artists that have come and gone and they want to keep things consistent. I don't know if it's fair to say that the style is popular - it's kind of a chicken or egg debate: is everyone using the style because the market is speaking and this style is what people want, or is it popular because it's the only thing that's coming out and your options are consume it or consume nothing (I think it's probably the latter, but I don't want to put words in people's mouths). As such, it's an appealing bandwagon for uncreative people to jump on - it's easy to get a large number of panels drawn fast so faster and more frequent uploads (which the YouTube algorithm loves), stuff with the style still gets views despite how oversaturated the style is, plus there's the fact that it's taught at a lot of big US animation schools, most notably Cal Arts (again, because it's easy to teach), so it's also got some clout. Basically the only channels I see that don't use that sort of style (at least of the stuff that use their own illustrations) are the weebs using that kawaii chibi style, which is a whole 'nother can of worms.
