The game has been done for years now, they're just running out of money, and figured its time to start up the hype machine again so they can sell some copies.
Simple economics. =P
Also, I think we all know that if we got our hands on the initial copy of DNF, the one that was almost done but they for some reason decided to scrap it, even today, we would make love to that game. If you watch that 2 1/2 min trailer that was posted, you can't even say it didn't look good.
I'm a firm believer that 3D Realms will make one of the most epic games to have ever graced our gaming lives.