There's only one reason; no more financial investors. 3DRealms used to be as independent as corporately possible :P , especially after the success of Max Payne (2001) combined with the previous 3DRealms games, they were easily able to develop for years without any additional investors. Those early 5 very happy years have ended about 4 years ago I think, meaning they've mainly been dependent on financial investors for the last few years. I'm guessing with the economic crisis and all, they just couldn't get anyone to invest in a product with such a notorious name within the industry.

Again, it's a very very sad day. I'm hoping some publisher will give the game yet another chance though. Prey was good, Duke Nukem 3D was awesome and even Duke Nukem Manhattan Project was great fun to play, however much it had budget-game written all over. By the looks of the 2001 trailer and the 2009 trailer, the game has come a long way.

Even if skeptics (read:ex-employees) always have claimed these trailers were demos to showcase the game idea that existed on paper only, meaning no real levels were made, no real gameplay even existed. I sure hope this ain't true as it would support yet another conspiracy theory involving Epic and 3Drealms Unreal engine support,

Cheers