> At least for me, custom games in Reach are dead. They were so from the very beginning, actually. Not that I dislike custom games - quite on the contrary! But somehow everyone I encountered was far more interested in playing matchmaking, doing challenges, and generally earning credits than they were in just playing funny and/or competitive custom games.
Unfortunately it has to do with Matchmaking being promoted more often than any other game mode in Reach, with Campaign and Firefight sometimes getting some love here and there, and as you mentioned it has do do with challenges and credits. With Reach, there is no incentive to play customs other than for the sake of fun, which is all good, but due to how many people think and how the system is set up for matchmaking promotion (achievements, credits, challenges, commendations), they want the biggest pay off to get them farther within their Reach career (or rank).
Granted there are those who have other reasons to play matchmaking (it is their idea of fun for example), but again I think it has to do more with promotion than anything. If there were no credit system, commendations, challenges, or achievements relating to matchmaking, I think there would probably be less matchmaking in general and more people playing customs, as they get the same amount of in-game reward for playing either matchmaking or customs.
Of course that does not mean people would go completely cold turkey on matchmaking, as sometimes people just want to play a game without having to worry about setting up the entire game or dealing with those kinds of people who only want to play their kinds of games.
Thankfully there are groups like Forgehub and XForgery for customs. Granted that they exist more for presenting maps than anything else, but still they can function as a place to play customs. If these groups did not exist, I can only assume that customs would have less population than the least played playlist in matchmaking.
EDIT: Another factor that I just realized is that there is no search function for customs. I am not talking about something like matchmaking, but something that allows players to find open lobbies for games. That could also be matchmaking is favored over customs.