@DiegoPalacios provided some key ideas on why these things happen. They all link back to the efforts of the creator and how they're getting their work out there. I think the trouble with only seeing from the outsider perspective is that we only see that and not the behind the scenes.
Some comics that gain a lot of popularity with few pages and are seemingly popular "overnight" have creators who've been at it for a while. They've created other stories, some that probably weren't popular, and learned from the experiences to improve themselves. Then there are creators who were able to bring over fans they've earned doing other works (fanart, fancomics, another comic, etc.).
At the same time, the comics that are gaining readers slowly might not be doing the advertisement they need or aren't really engaging their audience/other creators. They have the art, the story, but no one really knows who they are or have seen any of their other work. It might even be hard to find them, and if you do -- they might not have a lot of art/creations posted.
Even with this, skill isn't always going to be the thing that drives people to you. There are a lot of professionally done comics that are online that have little attention. At the same time, there are comics that are roughly drawn that are getting more attention.
It really depends on the efforts of the creator to get out there (and maybe a little bit of luck along the way). When you put yourself out there, people will notice. After all, even if you have work DaVinci would love, if no one knows who you are or if you don't have a presence, people won't know what you're offering.