Aye, I only RP on Discord, so I can give some info on that.
So a lot of people just prefer Discord because of a lot of things, and these things make everything a lot more enjoyable for the people who seek it.
1. They don't like other people telling them what they can or cannot pretend to RP. Generally people telling other people what they can or cannot do in pretend land leaves a bad taste in their mouths.
2. Discord makes organization a lot easier and accessible in comparison to a forum. Forums for RP are super outdated, but still applicable if it's preferred.
3. Discord has pins and can exchange information quickly with the use of notifications and many, many different settings that a forum can't provide.
4. You can have multiple RP's in a single server, and you can have many partners all privatized on a server for yours and their security / privacy.
From my experience, it's a very small population that are as you've described. I believe a part of the issue is more or less the mood difference between contacts. Some people contact others while in a mood, and when they don't get re-contacted during that mood, they seem to vanish. This is why F-list is useful, because it's got it's own advertisements, and people can contact each other while in that same mood. I personally find that a lot of the F-list staff suffer from god complexes or stalker issues, enough for me to purchase a VPN to avoid some of their staff from stalking me. That's an additional + with Discord though, you don't really need to deal with stalker issues or abuse of power, you're your own responsibility on Discord.
With Discord however, you can make your RP play-land a beautiful and well designed piece of art. Here's an example of what I would consider to be a well orchestrated
Discord for RP! This particular server has 2 active RP's, an OOC section, and a FC section. This is why I prefer to take F-list points of interest from their client, to Discord, because of the tools that Discord has to offer.
Hope this helps! I'm sure that if you will find a partner that fits what you're looking for.