See, Bungie is looking out for you. In order to provide everyone a chance to experience the campaign without it getting spoiled by:
a) A random 12 year old yelling the ending into your ears.
b) A random deep voiced adult man yelling the ending into your ears.
c) Any random people skipping cut scenes…unacceptable!
So, as of right now, campaign matchmaking is disabled in order to prevent all of the above scenarios but Bungie says don’t worry, in it’s October update, the feature will be included along side some general bug fixes. See, you guys thought Bungie was gonna be corrupted by it’s new big boy publishing deal. They got your back.