The developers agreed to end the game in order to give the ending substance and clarity. By ending the game this allows the ending slide show to explain the effects of the players choices regarding different factions and settlements within the Mojave. The choices the player has made and the final battle at Hoover Dam are meant to have drastic effects on the Wasteland, with most resulting in an entire faction being forced out of the area. Allowing the player to continue after the Battle at Hoover Dam would demand many changes within the Wasteland in order for the ending slide show to have any depth or meaning. That being said, the developers created an ending in order to let the player feel that their choices had a real affect on those within the Mojave.