DNF is a classic cautionary tale of "the perfect being the enemy of the good". The game was announced in 1997 and was pretty close to being finished multiple times, but the head of 3D Realms (George Broussard) kept demanding the game be scrapped, usually for very trivial reasons, and work on the game started back at square one. This kept happening until 3D Realms eventually ran out of money and the company was bought out by Gearbox Software, owned by Randy Pitchford, the guy responsible for other infamous trainwrecks like Aliens: Colonial Marines and Battleborn, who then released a half-arsed, slapped-together version of the game in 2011 - nearly fifteen years after it was first announced.
Broussard wanted the game to be the best game ever. The game was first developed on the Quake Engine, but when Quake II came out he scrapped all the work up to that point and started again on the Quake II Engine. Then when Unreal came out he again scrapped all the work and started again on the Unreal Engine. Then Halo came out, and... you get the idea. The result is that DNF ended up perpetually playing catch-up to the competition.