You realize It's only been made by 2 developers right?
From 1997 - 2008 3D Realms was developing the title, but they were always updating the game with new tech... so it never got done.
Then 2K sued 3D Realms for not finishing the game... 3D Realms filed bankruptcy... sending the game into development limbo
Then 2K announced that Gearbox (developer of Borderlands) was going to FINISH what 3D realms started, since most of the guys at 3D realms got jobs at Gearbox
So really It's just a different name... but the people behind it are the same!