RPGM is a highly limited engine. I mean, there's a reason it's practically nonexistent beyond a few popular adult games and JRPGs. Ren'py is are far more accessible and user-friendly, and has the ability to grow alongside any given dev as they get more experienced. It was a smart move for the dev to go the Ren'py route as he/she won't be closing his/herself off with a limited engine.
maybe as far as coding goes. unless you want an actual game. there is a reason that people cry about sandboxes in renpy, because it is tedious to constantly just click through menus after menus. renpy is fine for a visual novel, ie reading a story with pictures. renpy is awful when it comes to every other style of game. platformer? its limited to not being able to do it. seems like a pretty big limitation. old school jrpg style? forget that, you click to the next screen. end of story. anything with moving parts that doesn't involve clicking on a spot and a timer? completely out of the question. I have seen reasonable things done with renpy to make it appear more like a game, and a few that were actually enjoyable. if you want to read with sexy pictures in the background, yeah, its great. if you want to walk around, explore, find events, maybe some combat, do an action rpg Zelda style, I even saw one that I had great reminders of SOM from back in the day, you don't use renpy. rpgm can mimic game styles its not designed for. renpy does not have game mechanics. rpgm can even be used for a pure visual novel, where you walk a bit instead of click through 9 rooms to get to the next progess point. seems like one has a lot more limitations than the other. I would argue that the reason that seems nonexistent (it isn't btw, there are a ton of games that use rpgm, just fewer showing up here because of all the crying from people who just want to crtl through to the next picture gallery) is because of the crying... well, because of the thing I just mentioned. all I can say, in the end, is I have played a pile of rpgm titles that all have their own unique feel, and a pile of renpy games that all seem like clones with different pictures and stories. not everyone puts the effort in to make a good game, and rpgm suffers there. renpy it doesn't matter, because the game is going to feel the same anyways.
enough of my ranting, to each their own. just hope that the developer sticks with the feel of the game as it originally played out and doesn't simply switch to renpy so everyone can turn on skip unseen text, ctrl through, and screw the rest of us that actually enjoyed the game as it started because they didn't want to download a cg rip instead.
good day, ma'am.