In my opinion, I don't think it's really possible to pin the issue on 1 or 2 things. I think it being in beta for so long certainly hasn't helped it at all, as I know there are some people who just won't play betas. Along with this, there has been a lack of advertising as, I think, they want to get out of beta before they do anything in terms of advertising/publicizing.
I think being on Steam would help a lot, but keep in mind a complete asset recreation is a big under taking. While you could probably get away with not redoing the medium tank, since it's based off a real life tank, the others would probably all need to be done, and the team lacks 3D artists.
I've proposed changes such as splitting servers at 60 players, which is entirely possible and tested, but no big server seems to want to take the risk for the benefit for the community, which I completely understand. I don't know how the players would react either, some like 60 players, but a lot of them don't. So it's entirely possible that 10 of the 30 would just leave, which would probably result in everyone else going as well.