It might because that the imagination of "multiplayer" in users is not the same as official multiplayer plugin does.
Multiplayer games could be classified by server logic.
1. ajax + database ( php + sql ), players get data then play, finally save the result to server. This case has the minimal realtime interaction with other players.
2. websocket + ticking server, to create a continuous world, like MMO. This case need a central server to handle all logic of players and the game world.
3. action games in local network, like "Counter Strike".
Official multiplayer plugin could do very well in case 3.
Sure official multiplayer plugin could make a server to maintain a game world. But it only could be ran on browser , most of server engines do not need. (case 2)
And official multiplayer plugin could not handle database ( php + sql ). So developer will not use official multiplayer plugin in this case. (case 1)
Official multiplayer plugin is very suitable in multiplayer action games, and it is the best solution in action game for C2.
1. official multiplayer plugin is not suitable for all kind of multiplayer games.
2. official multiplayer plugin is not easy to use,
3. webrtc is not supported for all platforms today.
Developers said that they want multiplayer function, yes , they really want it. But multiplayer function is not a simple issue which could be solved in a single plugin.