Even today, with the campaign running as smoothly and as efficiently as it did, with 2 simultaneous streams going on, and with so many teams competing, the total run time of all the matches went for what, something over 2 hours? That number would be at least doubled if there was a double elimination bracket or best of 3.
Since this was a 6 day event and international timezones were in play, draining the endurance of all the competitors in one day isn't entirely ideal.
Plus, in my opinion, first to 5 kills makes the battle for the map titles more intense, urgent, and exciting!