I actually missed this comment. It's something I've noticed too, and it's left me scratching my head on more than one occasion.
But really, I think it probably does just come down to subjectivity. What you and I consider bad, tweens lap up like chocolate milk (that's what all the kids are drinking these days, right?)
I've seen some of what I would consider woeful artwork being praised as being the best on the site. I've seen webtoons failing just about every test of decent fiction writing being begged for in the comments section like it was crack. I don't think it's nefarious, I think it's just a case of young readers having poor taste and not knowing any better.
Some stories, for whatever reason, just hit the spot with the bulk of the regular Webtoons crowd, and once it all gets rolling, it can often start a snowball effect.
By contrast, on Youtube, you can go viral if you have something interesting to say that isn't being talked about in the mainstream, but presentation isn't actually all that important there.
I think it's all just about what the regular visitors to the site are looking for.