From what I've been able to tell, a large factor is how consumable a story is. Stories like 50 shades or other objectively "bad" writing will still become popular because of how easily digestible it is. A lot of premium or popular content you see on the site is like that, too: the plot is easily summed up in a sentence or two, things progress quickly and the writing is clear. There's a "low bar for entry", if that makes sense, and it's accessible by a wide audience. (Just to clarify, I don't mean that consumable = bad!! Often times the most well written stories have these traits too)
The more complex and nuanced your story is, the less fit it is for general appeal. Even masterpieces in writing and art may fail to become broadly popular for this reason. They also don't seem to like epic stories that will take years and years to create because those are inherently riskier. Tapas specifically also prefers full color and the mobile layout instead of traditional "comic pages".
My art is professional grade and I always get people surprised that my comic isn't more popular, but a lot of that reason comes from what I listed above. It's B&W, traditional page setup, epic-level long form, niche genre, slower production rate, packed with nuance, and frankly, written to be a graphic novel--not a webcomic. So if you can apply a lot of those same qualifiers to your comic or writing style, it's just not optimized for Tapas to make money. That doesn't make it bad by any stretch, but you'll just be struggling more unfortunately.