How exactly would they incorporate Forge into Matchmaking? If it were to pick a map one player had created, well, that guy has a huge advantage over everyone else as he knows how he laid everything out. I don't want to get owned by someone when playing online because of something lame, like he knowing where he stuck the shotgun. At least with friends, you can play on that same map repeatedly (and they're less likely to get pissed off), but in matchmaking you'd get stuck on a brand new map likely getting owned by someone everytime (assuming you're playing free for all), unless the map it chose happened to be yours.
There has to be a standard selection of maps to play on when playing online, otherwise people will stop playing because they'll get irritated so much because it's unfair.
themanwithnoname's law: As an America's sales or NPD thread grows longer, the probabilty of the comment "America = World" [sarcasticly] being made approaches 1.
Essay Pro