Remember when reach had forgeworld at first, no one ever used any other maps to forge on, that was not a good idea considering you want the community to play the heck out of multiplayer first, then campaign maybe, or vice versa. Halo 3 had amazing ideas in the maps, there was no official forging map until foundry came. People enjoyed forge, and made remakes in forundry. Then in reach, i have tons of friends who play nothing but forge or custom games. So multiplayer population goes down that way, its all in the release dates people, you dont want to compete a game like cod, it sucks yes but it will eventually end up like tony hawk.
Well with respect, the dropping population has a lot more to do with other game releases, and dedication of it’s players.
For instance, many people like to buy the newer games that come out.
Other people don’t (in the case this seems to be the biggest reason) like the core game-play imposed upon them in Reach.
Personally, I have few friends who even play this game, and those that do stay out of customs because the options are a bit “broken” as of now.
I will give you that forging seems to occupy the time of many people, but it’s been that way since Halo 3.
Some people would rather forge up maps, and get recognized for this, than to play MM.