One factor you may be missing in the prevalence of Isekai/LitRPG/Progression Fantasy novels on RoyalRoad is that the original purpose of the site was to translate Legendary Moonlight Sculptor, a novel about a Virtual Reality MMORPG called Royal Road which was emblematic of the genre. From there, people started publishing fan fiction of the story, then their own similar novels, and more. The audience for the genre never left and is still the largest on the site, even if now there are other genres as well.