As a player who really likes the Supreme Commander style of gameplay, there's a couple of reasons that have kept me from really getting into FAF:
x First one I haven't seen mentioned yet: There's a MASSIVE amount of meta that players take for granted, but is next to impossible to find out somewhere. Simple things like knowing which position on a certain map has certain jobs to do, standard movements, less-than-obvious counters to frequent strategies (ie. tele-defence for example). Learning these as you go is a huge timesink, since it requires encountering these specific situations and figuring them out or watching dozens of streams. And not knowing about it can often be instant loss. Which in turn promotes playing the same maps/positions.
x I enjoy the cooperative play of multiplayer not the competitive, so 1v1 ladder has never really been attractive to me. I'm curious to try out the new TMM sometime.
x I've always been surprised that there isn't some pre-start allies-chat/lobby thingy. This is supposed to be a strategy game and being on the same plan really helps. Communication within random teams is already often rather limited and as soon as you jump in the game you're busy with getting that critical first build going. For those who can do their build orders in their sleep this is the perfect time to chat strategy, but this is a high stress moment if you're trying to figure out what to do on this position on this map.
x There isn't much to promote finding a group of like-minded, same-time-online players to play with regularly. Which means that the 'community' you play in as a starting player is the entire FAF community, including its toxic elements.