I've got nothing against fanfic. Some of them are better than published novels. There are some amazing fanfics out there and the amount of dedication people put into them is incredible. I could recommend a tonne of fanfics that can make you cry, make you love the characters without ever having seen the series, hook you with amazing action and love stories
However, I agree that places like Tapas are to promote original fiction. AO3 and other places are for fanwork. One doesn't generally do well on the other, because you don't have the audience. People come to Tapas for original work and people go to AO3 ect for fanwork. That's what they're there for. I doubt many fanworks will last long here and I doubt there will be many of them get all the way up to popular because this just isn't the place for them, the sorting and access to fandom isn't there. (The cynical part of me says they're here because places like AO3 have too much fandom competition and they're trying to be big fish in small fandom pond but that's just me being a cynical bitch).