and what are mainstream games?

The ones on GOG, obviously. The rest are too niche and thus rejected.

Sorry if I came up ignorant, I'm not really informed about what is popular among gamers...

I think you;d have to be more specific. RPG can mean a lot of things. Something like the Witcher 3 or Skyrim is definately not niche, those are big, modern, famous AAA titles with huge player bases. On the other hand an old-school dungeon crawler like Legend of Grimrock, while still popular among it's target audience and financially successfull, is probably somewhat niche.

RPG mean "rocket propelled grenade", obviously. Or "Ruchnoy Protivotankoviy Granatomyot" if you prefer. :D

RPGs seem to be one of the most popular gaming genres. In fact, I'd place it second below FPSs. You can't even turn around without bumping into a game that might not even be classified as an RPG that still has elements inspired by them, ie sections of your character that improve over time through stats or points, or the ever popular crafting system that improves through repetition. There are genres that are far more niche but still seem popular within their circles. Flight sims. Grand strategy. Zachtronics style puzzle games. Even fighting games are still very niche. I'm not sure where to place the newfound "battle royale" genre ala PUBG, but certainly it seems like many of the above require a baseline set of gaming skills plus an intense study of manuals and how to videos to get started.

As with all games - accessibility, difficulty, general gameplay. Simple rules, action-based combat, open world where you can do anything, that will make a game mainstream-friendly. Old-school type gameplay, complex character stats and progression, slower and more tactical combat - that will make for a niche game.

At its most base definition, the more niche a game is, the less mainstream appeal it has, whether it's through character design, storyline/plot, graphics style, system framework for the gameplay, etc.

A game like Until Dawn for the PS4 has RPG elements but is more of an action adventure game; however, it's niche being in the horror genre as well as having a lot of Quick Time Events. In this situation, both the genre and gameplay style determine it's more of a niche title, and explain why it didn't have more success to this point.

I hope this helps, even though the game I mentioned isn't a "pure" rpg.

opps somehow cross threaded, anywho, if you think about it all games have a following, otherwise they wouldn't produce that style of game, someone loves shooters, someone loves dress up games, Yes TinyE I know you do.... if someone is willing to pay for it, I'm certain there is a game about it so you can't really classify any one style of Game as a niche game, well except for *cough* gayporngames*cough* but even then, it does have a following.

That's the problem, but also the answer to the question. Basically, when certain wildly popular game mechanics converge in what effectively is a "genre mix", it's going in the RPG genre box by default. So a whole lot of very popular games are called "RPG", and it's not a niche genre by a long shot. However, if you take 90s standards as the basis of your evaluation, what's been an RPG back then would pretty definitely be niche right now.

What I would consider to be the most pure of RPGs are definitely niche at this point.

The first signs of RPGs becoming more mainstream, back in the late 1990s, were really the result of the games, in a sense, becoming less pure. In terms of JRPGs (like Final Fantasy 7), you have games putting in a heavy focus on story over gameplay, with excessive amounts of cutscenes; this is also when minigames (almost all of which are not RPGs) became common in the genre. With WRPGs (like Baldur's Gate), the game has moved away from being turn-based, and you also see some rather complex dialog trees, which feel more like the sort of thing that would fit better in a visual novel or adventure game.

Even later, because people complained about the random invisible encounters common in JRPGs, developers started making the enemies visible, but had them move in real-time, which again makes the game less "pure". (One counterpoint here; the SaGa series has had visible enemies moving in real-time since the Super Famicom days, and that series is generally considered niche; as much as I adore that series, the visible enemies moving in real-time is actually one of the things I *don't* like about the series.)

So, "pure" RPGs, as I am calling them in this post, are a niche genre; those that adopt aspects from other genres, often at the expense of their RPG-ness, are the ones that tend to be more mainstream.

I was making a joke. GOG has a track record of rejecting games with the excuse of it being "too niche". It's happened so often that it's become an inside joke around here.

I though Opus Magnus is rare example of game rejected by gog... in any case, I missed the joke...