What sort of games are you comparing them to? Are we talking about porn RPGs or RPGs in general?
I'm sure some of the people who like RPG porn games actually like RPG gameplay, with things like map exploration, turn-based combat, and whatever.
There does seem to be this idea out there that the turn-based RPG as a genre is outdated. Commercially speaking (non-porn commercial games, I mean), you don't actually see them around too often whether in 3D or not--sorta like what happened to the point-and-click adventure game genre. I tend to think it's just a matter of personal preference. Indie game development being so prolific these days, it's hard to see any particular genre fading away completely like the point-and-click did.
Honestly, part of it is probably nostalgia. At least some of the games deliberately emulate the styles of older RPGs, and even the ones that don't will sorta carry on a bunch of conventions of the genre. It helps having RPGMaker and Wolf RPG running around--you wouldn't see nearly as many of these kinds of RPGs out there without those tools available, as they really lower the barrier for indie devs. I suppose if they didn't exist someone would've put together a framework for them in Godot or Unity or something.
At the same time, I'm not really sure why the "RPG" genre gets singled out for this. The 2D platformer is even older, for one, and in the non-porn arena the arcade fighter is only hanging on because of its competitive player base.
Just from personal observation, it seems like the Japanese (porn) gaming market is still fairly keen on them, although I feel like there aren't as many straight RPG-maker type RPGs coming out over there. Year or so ago you'd see several a month. I think the novelty of MV is wearing off, and devs are starting to try to do less RPG-like things with them.
