Because Bungie said so.....
http://www.bungie.net/fanclub/316529/Group/GroupHome.aspx
Firefight 4 Fun is a group dedicated to organizing Firefight games and tournaments to make up for Halo 3:ODST's lack of MM. Win big prizes and have a great time. I plan to have our first game on the Saterday after ODST's release, September 26, 8:00pm central time but I need to know YOUR schedule. Tell me what time works for you and I'll try to make it happen. Here's the link......
http://www.bungie.net/fanclub/316529/Group/GroupHome.aspx