An FPS is a first-person shooter. There are lots of multiplayer versions of them where teams of players work together to fight an enemy. An RTS is a real-time strategy game. It is where a player (sometimes two facing off) has a birds-eye view of a battlefield and commands nonplayer computer characters to do things like build bases, fight enemies, and so forth. For decades people have thought about how cool it would be to have a game that combines both. You could have a general player telling the units what to do, but the units are actual players! It almost never worked.
People sometimes want to follow orders. Other times, they don't. Whether you're in an office told to get a project done by a specific time, have a child demanding you buy them a snack, or you're a computer soldier on a battlefield with some other player spamming requests to fight an enemy/build something, we as humans hate being directed to do stuff. The actual military is designed to teach the importance of following a chain of command so that everyone lives, but a virtual battlefield is the kind of place where if an enemy shoots you down it just takes 30 seconds or so to get right back to fighting.
There are many failed FPT/RTS games out there and barely any success stories (I guess the, "Natural Selection," games). I myself used to play, "Savage," on the PC and maybe one time was in a match that actually worked with FPS folks listening to the general-type player. This hasn't stopped studios from making new ones, with Bohemia planning to give us, "Silica." In a twist that makes me hopeful everyone playing together will be able to a commander and boss your group of NPCs around or switch to a first-person view to fight and drive around vehicles. That could be fun if it works and at some point, a game has to nail the format in more ways than not.