RoyalRoad's origin is as a site for fanfic of a Korean VRMMO novel, Legendary Moonlight Sculptor. Up until a couple years ago the site was RoyalRoadLegends (domain royalroadl.com), named such because the game in the LMS setting was named Royal Road. Anyway, the site stitched together an abomination of blogging and CMS frameworks to make a fairly functional site for posting webnovel stuff and invited its members to post their own stuff, and so it all started with stuff like what they came to read, original VRMMO stories. LitRPG is just a half-skip away from VRMMO as a genre (the fundamental difference being that VRMMO knows it's a game while LitRPG has game elements in a fantasy or very soft sci-fi setting) so it was a natural fit to have new original LitRPG stuff there too. Those were the only things posted on there for a good while.
Then translations of xianxia/xuanhuan/"cultivation" novels took off and people started wanting to do originals of those. Wuxiaworld, the site that really popularized that stuff, teased about having an originals section but the site owner ultimately didn't let it happen (a combination of him wanting to be an absolute control freak and also him keeping too many plates spinning and losing track of all the promises he was making), but they had a contest partnership with what was then still RRL to write fanfic for one of the popular series on Wuxiaworld and RRL added tags to support discoverability for that kind of stuff. When nothing panned out on Wuxiaworld itself but people wanted to put out their own stuff, a lot of them decided to do so on RRL. That's when the base officially broadened from VRMMO/LitRPG and added Cultivation novels as a genre, (and it's also when the people running it started gearing up to run a business rather than just a hobby, refactoring the publishing side of the site to not just be a CMS/blog frankenstein). The core audience of the site to this day attaches to those genres.
Eventually the site owners were able to secure the royalroad.com domain and since they obviously weren't just about LMS fanfic anymore decided to drop the L. Then Wattpad, which is just super saturated at this point, had a kerfuffle involving them shutting down their forums and basically destroying discoverability for new writers, so a lot of them migrated over to RR and really muddied up the genre list. The core reading audience there, though, remains the "power progression fantasy" audience: LitRPG, VRMMO, and cultivation stories get the most eyeballs. And that shit's a decent income. The top-tier Patreon and publishing types have $5k-$20k/mo. Patreons plus their Amazon incomes. And they just churn out popcorn fic month after month and keep that income. It's pretty bonkers.
(I know this stuff because one of my online gaming friends had a top-rung novel there once upon a time and rambling about it all was a hobby of his when he could fit in the time to actually sit through a session of a 4x game with us. The pressure of keeping up on it all eventually made him flip out and nuke everything from orbit and watching that fallout was pretty popcorn-worthy too.)