2 team Frontline can lead to games where the winning team happens to be stacked (premades or just better players) and they end up stomping the other team, leading to base camping and people giving up and afking in spawn. To counteract this, you would need comeback mechanics which then would make close games feel annoying because getting a slight lead at the wrong time may actually put you into an unfavorable position. The 3 team system can counteract this, where the 3rd team can punish the stacked team if they try to gang up on a weak team.

Those games where people give up immediately are "quit moments". Even if it just happens occasionally, the experience will be traumatizing enough that they refuse to engage in the mode anymore. This is not good for a mostly casual mode that requires large amounts of players to queue. We saw this in the previous iteration of shatter, where a skilled team could absolutely stomp the other teams with not much risk of getting flanked or losing objectives. These matches, on the losing side, led to a lot of agonizing or even toxicity as people sit in spawn and insult their teammates instead of playing. You could maybe argue that the random map gimmicks from CC make more sense in frontline than in CC itself, which is designed as a hardcore competitive mode.