MAT (153175) on May 26, 2016
More to the fact that in the past we only had add-ons, not DLCs, and they are same thing here. For example StarCraft: Brood War is add-on, it has its own credits alright. In the past add-ons were much larger and almost like the base game which required base game to run and was usually expansion of the base game. Such add-ons still exist and we just mark them together with DLCs, but they often have a large team and new set of credits and whatnot.
Also, many episodic games are all DLCs to the first episode, yet they are just as long and development heavy as the first episode, so sure, they have their own credits and whatnot.
Smaller DLCs like new outfit or new weapon or new dance music or new multiplayer map and such are probably the focus of this new scheme.