The long term value for an RTS is with the multiplayer. The problem is that the games are incredibly demanding to play. It takes a lot of practice and experience to get good enough to get off the “bottom rung” of the online ladder, just to be able to reach close to a 50% win rate.
Many gamers find this too stressful to deal with. They end up quitting the game and moving on to something else. This causes the “bottom rung” to move up in skill, as the weakest players leave. Since winning in these games is zero-sum it gets harder and harder to win the more weak players leave. Eventually only the most dedicated players remain, making it extremely difficult for new players to break onto the scene.
The other aspect to RTS games, the single player campaign, takes a lot of effort for the developers and designers to build. Unfortunately it has very little staying power. People play through the story maybe once with each faction and then either try out multiplayer or move on to a different game.