First, the combined billing costs for all the SKU's is around $80,000. If the publisher doesn't think they can make that money back right away they don't release DLC. The timing of other more popular releases or if the previous DLC came out too recently will have an effect. Also, Microsoft has direct access to the number of units sold in all DLC and how many people are playing the game at any given time. If they aren't pleased with the numbers they pressure studios not to release anything, including patches. An extreme example of this is Duke Nukem Forever. Gearbox was able to release a patch for the PC version but not the console because the certification process and high cost is blocking them every step of the way.