The overrun is a relic of the Monday Night Wars when Raw and Nitro were in a dogfight for ratings. This was before DVR, so strict adherence to timeslots wasn't a concern. And there wasn't anything on other channels at 11 PM on a Monday night to be worried about unless the MNF game was going down to the wire. Even after the war ended, the overrun was usually the highest-rated part of the show because viewers had been conditioned to expect the major angles to take place then. That stopped being the case after Raw expanded to three hours. By that point, though, the overrrun had been around or so long that people who grew up on it entered the business and figured that was just the way things were done because it was all they knew.