I started playing again a few months ago so while I'm not new anymore I certainly was then and remember the experience.
I think there are a few factors contributing to low player retention but the most important is FAF's very steep learning curve. It's often said that the best games are those easy to learn but impossible to master. Imo FAF is very hard to learn to the point that it's almost overwhelming.
In terms of team games, players come to FAF having seen replays of GCs dueling it out with chickens and com drops and close tactical games and that's what they want to replicate. What they are then faced with is noob gap games where there is usually lag, nothing happening for 20+ minutes and suddenly one team completely winning.
I took the route of ladder and it was frankly terrifying and stressful up until about 600. Getting past the first few games of ladder is incredibly daunting as you have no idea what to really do and can get matched against high rated players.
Tl;dr FAF looks really cool on youtube and then people join and find it very hard to get good enough to have interesting games. They then also have to deal with "no greys" "no au/no ru/no eu/" lobbies, endless boring gap games and often a not particularly nice player base.