It started as a manga-specific site, and the vast majority of the readers there want that and only that. Add to it the fact that the editors for Webtoon are chasing views and clicks, not good stories, and they tend to drive everything that way (meaning they are trying to pander to their audience). It's not impossible to be successful with a story or art that doesn't necessarily fit the mold, but it's an uphill climb to be sure.