Why wasn't it included in the main game? Because the content wasn't nearly complete by the time the main game was locked and into QA, certification and manufacturing.
That doesn't make much sense to me. DLC also has to go through QA and certification on consoles. PC doesn't have certification. If you could finish the DLC content, test it and submit it to certification in time for the release of the game, couldn't you have included it with the game?