Because primaries are underpowered. At that level of play, the TTK is simply too long.....and there is simply too much time for a good player to run away from a losing gunfight. Which means that 1v1 primary duels are a waste of time. No one ever really has to commit to the fight....and if you do....it just makes you vulnerable to getting teamshot. Which is why Comp and Trials is about special weapon spam, and ability spam. Those are the only weapons that kill fast enough to force a result from an encounter.