Shooter was 300+ ft up (32nd floor) and 400+ meters away, reportedly. Handguns were useless unless you could get to his room, which no one in the concert crowd had any prayer of doing.
even if the cops at the music venue had rifles, they are at a tremendous disadvantage due to elevation — caliber is the least of your problems.
The trick is to get someone on the 32nd floor.
Another example is the University of Texas Tower shooter int he 60’s. Same scenario, but the police actually fielded rifles which were useless against the sniper until they got someone in the tower...