Acording to Seteamspy this game sold 2,983,077 ± 45,448 copies, so, why didnt you make an expansion?, a lot of people would have bought it
Steam does not release the information to anyone.
Generally speaking... mods. It's tough to justify developing a paid expansion when people have pretty much all the tools available to import any additional content right back into the base game. Beyond that, it's just not something we've pursued.