Legally there's no such thing as abandonware. So, even if there wasn't a live entity that owns the IP (there is), anyone developing a game for profit without the copyright holder's okay would certainly be in the courts and would lose (given that there's a live entity holding the IP, that'd likely be the case even if the game was not for profit).
So anyone developing a game is faced with the option of acquiring the rights or designing something similar. It might be worth acquiring the Dune rights for a Crusader-Kings-II-in-space game, but I'd assume the small market for that sort of game means that there's not a lot of profit to be spread around on acquiring rights. For something as relatively unheard of as EFS, companies such as Stardock would just make their own universe up.
Love the universe though, even if the SP game was just too unfinished to be enjoyable after a couple of plays.