Why did it fail? They outsourced the multiplayer component to a company that only concentrated on creating the multiplayer content.
And all 3 DLCs so far were made for the multiplayer, and they even created that snapmap feature, that makes it easy to create maps for multiplayer ... and although all this effort was invested into Doom and Doom was a commercially successful title and has a big playerbase the multiplayer failed in the end nevertheless. They even created a multiplayer feature where players can transform into a demon and hunt players down. Similar to quake they concentrated on Deathmatch like a typical Arena fps, but although it was the newest fps by id software it won no players over. A lot of people anticipate the new quake champions now, but why wasnt Doom 4 a worthy contender back then?