I think part of it was the Sony stipulations for revenue sharing on crossplay, but also, given how much else in the game seems rushed or poorly implemented (no Quickplay, no dynamic maps, difficulty levels based on bullet sponging and ammo reduction, etc, I'd guess that it just got cut because of time, or the Devs didn't think it was necessary.
This game isn't quite a "case study" in bad game development, but it's close. No Quickplay is probably the single greatest factor for its plummeting Player Counts. No game should lose 90% of its player base in a month. But people got tired of playing that first mission over and over, or not being able to find human teammates through matchmaking.