If you ever had to introduce new people to Supcom or FAF you would know exactly why. Me and a few friends have been trying to get our other group of friends into supreme commander. We play with each other almost 3 times a week (about 4-8 people) but only two of us (me and another guy) has played FAF multiplayer.
The campaign teaches you everything you shouldn't do in multiplayer. Almost everyone in the campaign just turtles up and techs. Someone could've have played Vanilla supcom for 200 hours and have no experience fighting with t1 units. The biggest hurdle in FAF is unlearning everything singleplayer taught you.
Vanilla Supcom FA doesn't have a basic tutorial since it's an expansion. The first mission just drops you into the action with 25+ units & buildings to work with. If you never played Supcom 1 you get nothing.
The FAF tutorials are NOT HELPFUL AT ALL for onboarding new players. The first ones have you memorize map specific build orders, if someone doesn't even know the basics then memorizing a 15 step build order is intimidating.
The learning curve is there, but it's exacerbated by the fact the knowledge is best found through outside guides rather then vanilla gameplay. Only someone already invested into Supcom will binge YouTube Supcom tutorials or read a 3 page guide. If you don't have a friend to play with, then you are stuck with the public lobbies. That's pretty intimidating for a new FAF player.
New players don't know how big the game is supposed to be (partly because they play it like other RTS's). They need to learn it's ok to build 20 factories and to expand outward.
I think some sort of "casual matchmaking" could go a long way. It could also have an option for game modes other than 1v1.