Like publishers, websites tend to vary in their "popular" sellers. For Tapastic's paid comic section right now, a lot of it is either gag-a-day, or romance (with the open free platform on the desktop being BL and gag-a-day). Whereas if you look to LINE, a lot of their paid comics are long-form adventure/fantasy, with some slice of life comics here and there, and of course, gag-a-day (most places love gag-a-days because they're simple, bite-sized, and relatable - not everyone likes romance, but everybody loves comedy in some form or another.... it's way more flexible in style and timing and such, and therefore makes a lot more money because it's a lot more of an open market).
Romance is one of the dominated genres of novels, alongside YA (young adult), and these two typically go hand-in-hand (even intense YA novels like Hunger Games and The Maze Runner had romance as an element to the entire story). Fantasy novels haven't really been in huge demand since the 90's/early 2000's, and that's probably because YA/dystopian novels have booted them out. No one really cares about world-building or plots based off 4000 year old histories, they just want something relatable while still maintaining some fantasy. Like yeah, the Hunger Games takes place in a dystopian universe where locations are split up into districts and every year they send kids in to fight... but that's where the relatability comes in. The main characters are all kids/young adults, so the readers can still relate to them on a grand scale even though it takes place in an unrealistic setting. No one can really relate to Gulvin the Dwarf, at least not on as grand a scale as YA novels broaden out to - you really have to grow up with that sort of stuff to enjoy it (with things like D&D, Game of Thrones, Lord of the Rings, etc.) whereas with YA anyone can jump in or find a story within the genre that they can enjoy.
I'm digressing quite a bit though lol Tapastic just seems to have its target audience and they know what they can sell. Their goal with the novels is to make money, after all - and they can't if the stories they pick aren't going to draw in money. They might be able to draw in money with that super extensive fantasy series, yes, but not nearly as much as they could pull in with the story about a girl in high school who's just trying to fit in, all while falling in love with an unsuspecting guy. Like how Scholastic's biggest market is kids/teenagers. Could you see Scholastic publishing Stephen King's works? 
So that being said, no, Tapastic might not be the place for your novels. They have to sift through so many applications and works on a regular basis though, so I mean, complaining that you didn't get in isn't very productive. Just keep submitting your work to as many places as you can before they take. Even J.K. Rowling got rejected several times before Harry Potter was finally accepted and became one of the world's most successful novel series.
It should be noted though that, according to forum posts and what they've told people who submitted their work (myself included), Tapastic IS working on an open platform for their novel works - so like comics, you can just submit your work to the platform for free (you won't get paid for it obviously, but I feel like adding that will create a lot more opportunity for people who are more writers than artists... and there are definitely a lot of people out there who are. There are a lot more people who are artists who became writers than there are writers who became artists... it's just one of those rifts that only combines if you start with being an artist vs. being a writer).
