Making it will affect current game one way or another.
first of, how much work they would have to put to make it, and maintain
Secound, if they would gona do it as br, it would need differend balance changes than main game, again more work that would not benefit main game just take them time to do.
Base on how fast they made current heroes its bad idea. It take them long because of how polish they are, if they would start working on differend set of heroes with differend animations, look, weapons etc, it would take them so much time that it would affect main game a lot.
Lastly, remember what was going on when fortnite was created, epic games killed off other projects, and turn down idea of making fortnite story mode a lot at time just to make battle royal cash out more. It could happend in this dev team too.