Rhythm's hit pretty much everything above. There was just one little thing I noticed last night that might be a hiccup with your submission, L Nimbus. I'll quickly mention it here in case it's part of the decision.
In your synopsis of TMP on Royal Road (if this is the submission you're talking about), you have the following paragraph:
"This is a semi Parody of the game Exiled Kingdoms. Several concepts and monsters are drawn from there. if you like this story, i reccomend checking it out. it's free to play on Android and available for full purchase on PC. It's an Offline RPG that i've poured many hours into, and helped me partially when writing this story."
Now, on Royal Road TMP is listed as original fiction, obviously. However, the wording you have in that paragraph can make it seem like a fan fiction or derivative fiction. You also kind of do an advertisement for Exiled Kingdoms in there. It's great that you're sharing your creative inspiration and want to share the exposure, don't get me wrong.
If you posted the blurb you have on RRL straight into the submission info, that might be why a member of the editorial staff was hesitant to allow this through. There have been other rejections recently that kind of came across as fan-fic whether or not they were.
As far as large established audiences go, TMP is pretty big on RRL. You've got a solid readership. I just don't know for sure if that's the main reason. WFG lets on game-lit stories, some fairly dark stories, and some religious-themed stories. I don't think it's the actual content that caused the rejection. I think more than anything, it was something technical. Of course, I could be totally wrong.