Mainly because it's very unfriendly to newcomers, with complex terrain causing newcomers to be shot down before they even see the enemy, or to be "stolen to death" before they can even enter the door.
Basically, it is difficult for newcomers to R6 to have a good experience before level 30, and with the different skills and tactics of each R6 officer, it takes a long time for newcomers to get used to the game.
However, since the skills of each R6 cadre are not quite the same and many of them are solo, players should still have the possibility to rely on their own shooting skills to win, even in the event of a one-on-many handicap.