Limitations. The reason Halo isn’t meeting Skyrim numbers on this end is the inability to create a single player experience.
Imagine being able to mod the campaign. Let the idea stew. Realize Bethesda has only done as well as it has cause of the ability to create unique experiences.
When I think about forge, I lament the ability to use enemy AI and create quests. It’s purely for multiplayer experiences.
There’s a gem in Halo but right now it feels more like a blood diamond.