If Destiny 2 had a million players in-game everyday, having Match Making on everything would make a lot of sense, there would be enough players to populate all those lists. As it is, having Match Making in every activity would spread players too thin. You'd end up queuing into an activity and sitting there for ten minutes, waiting for two other players to queue into the same activity you chose. Instead of people complaining about lack of Match Making, we'd have players complaining about empty queues. Funneling players into a limited set of activities is the reason Match Making works at all, because there are more players in those activities to match with. If you're the only player queuing for an activity, if there's nobody else there to queue with, then Match Making just wouldn't work.