Most likely Bungie fell behind schedule (possibly due to whatever was going on with Marty / Joseph Staten), and Activision decided to shave the game down in order to meet the release date. This is not uncommon practice with big publishers, and some content originally planned for the game may end up as DLC as a result, or be omitted altogether if Bungie doesn't have the time/money to finish it when they need to focus on other projects.