The problem isn't simply that OpenSim has too few people. It's rather that too few OpenSim users even care for immersion and character play/light roleplay. (Shameless self-advertisement: I've created a group for this.
https://opensimworld.com/groups/1069 ) Some reject it outright, some even insult those who do care as idiots who are "doing OpenSim wrong". For many, OpenSim is nothing but a 3-D chat in a fancy environment.
You can see it everywhere.
You can see it in the avatars. Male avatars clad in heavy black leather on tropical beaches, just because they always have to look badass. Female avatars in micro-minidresses and 6-inch platform sandals with 12-inch spike heels both on sandy beaches and on Christmas sims covered in snow and ice, just because they always have to max their sexiness out. Okay, partly also because many popular freebie sims simply don't offer them anything that'd work better. Also, avatars that completely change their hairstyle or even skin tone on a daily basis.
Also, if you walk away from a party so that people won't see you teleport away, you're a weirdo. You're expected to do as everyone else does and teleport away right from the dancefloor.
You can see it on the sims. Paths that are purely decorational and completely useless because they can't be walked, but the sim owner doesn't know because they've never walked them themselves, and they don't care because they don't think anyone else would walk these paths.
Having to teleport over distances that'd be short enough for a walk of a few seconds, but the sim is decorated in such a way that it's impossible to walk. Or at least having to walk through lots of flowers that grow in your only possible way between the landing-point and an important location on the sim because the sim owner has literally never walked around their own sim even once and always flies everywhere, and visitors are expected to use a teleporter for a distance of, like, 50 metres.
Walking actually being required, but with a slope in the way that'd even be too steep for stairs in real life. And you don't have any stairs in-world, you have to crawl up that hill.
Beaches on which you can dance, play Greedy and fuck, but swimming is impossible because the sim owner doesn't even take it into consideration. Also, changing clothes in an immersive way is impossible because the sim is General-rated, but there's no place where an avatar can hide to change clothes.
Personal homes in which all added furniture is only for sex and/or BDSM, but there's not even a working bathroom unless it's pre-installed in that house.
Almost all restaurants, cafés etc. everywhere lacking scripted furniture. Sometimes any furniture. They're just fillers on a big urban-style freebie sim. Bonus points for having to walk on green grass to get from the sidewalk to the restaurant entrance.
Park benches freshly ripped from SL, but unscripted. Although several park benches with working sit scripts, even legal ones, are readily available in OpenSim. But unless it's sex furniture, who needs sit scripts anyway, right?
Islands completely surrounded by rocks with not even one measly wooden landing pier. How are people and supplies supposed to get there and back again? With a helicopter landing in the sand between palm trees?
Most OpenSim users don't even know that something like immersion exists. And they clearly don't care a bit.
Send them to one of those Cuteulala Artis amusement park sims. They won't ride the coasters and the waterslides and all that. Instead, they'll complain that they can't copy anything. This is also why the rides are often broken on these sims: Nobody ever tries them, so the sim owners don't know.
And RP sim owners are wondering why nobody comes to roleplay?