Why did the Halo 3 online population decrease dramatically?
because now to play halo 3, you will need all 3 map packs, Heroic, Legendary, and Mythic. Most people who don’t have the money were okay with the heroic maps and the original map cycle. But before ODST came out, Bungie adjusted the matchmaking options so that you will need the map packs to play. Now people can only play lone wolves, Squad Battle, Social Slayer and Social Skirmish. It really limits your options.
