The problem is/was the learning curve. And of course the UI. I really hope the Steam version is done well and not just one of those graphics packs that you can install on Dwarf Fortress.
It's a proper UI and it includes in game tutorials and help pages to explain the basics of the game as well as making the game much prettier than any existing tilesets IMO.
DF is also not as 'hard' as most people think, if you start your fort in an easy location you can quickly build a self sufficient colony inside a hill or underground and make it inaccessible to most threats once you understand the basics of production in the game. And with the UI upgrades managing your dwarfs' moods is a lot easier too.
The only thing that has been preventing me from being an annoying df evangelist for years has been the horrible UI :D
