Paraphrasing the thought process for switching from PvE shooter looter:
the original game plan was some kind of session based pve raid with multiple squads converging to a central point and fighting a boss
it was a lot of fun moment by moment but it wore off very quickly game by game because there was no sense of progression or meta game
the switch couldn't just be slapping pvp on top of the existing pve build, they had to rethink the whole structure and balance of the game
(which also explains why they swapped the release order with The Finals)
they wanted to keep the exploring of the environment loop (known internally as milestone 12) which was the most engaging part, hence the slower pace compared the early builds
Paraphrasing the thought process for switching from PvE shooter looter:
(edited for formatting)