IMO for two reasons, separated by player type. Casual players were turned off by the grind, and experienced players (read: players of former games) were turned off by the dumbed-down mechanics as compared to previous games.

By the latter I mean mostly the decision to eliminate base play, i.e. gens/turrets/invs mean basically nothing because you spawn in your load out. Also, the jetting/skiing physics are kinda wonky, in ways that I think negatively impacted gameplay.