Expanding on what Vance said, the biggest hurdle original roleplays have is it requires players to learn a new set of lore and all the details of a setting, like races, locations, important events, and all sorts of other stuff including rules about technology, magic, factions and relationships. It's kind of an uphill battle, and the best original RPs keep things accessible, which can often mean using beat to death tropes, like a fantasy RP that uses elves, dwarves and humans verses orcs and dragons kind of deal. If your potential players are spending more time trying to keep your overly original setting straight instead of focusing on character development or roleplaying, you may be in trouble. It's kind of like if you join a fandom RP you have never seen the source material for, only there's no wiki to save you.

This is a part of the reason I prefer fandom RPs because people already know the setting and most of the lore, all they have to worry about is making a good character and playing. The thing some people don't seem to realize is that somebody running a fandom based RP can be just as creative as somebody making a totally original world; they still have to come up with a captivating plot and keep players invested, which requires quite a bit of dedication. 

Plus, it's fun to tell stories in a world you love to flesh out that universe a bit more.