Because like I said, the entire leveling experience of Legion actually
had nothing to do with Demons. Broken Shore was the first and only place where we encounter demons, then as soon as we start questing on Broken Isles it's all about the Pillars of Creation and obtaining them from Old God related villains like Xavius, Azshara and the Drogbar king. And from there, we had plenty of locations that were directly tied to POTM and Wardens, like the Vault of the Wardens or the Temple of Elune in Valshara, as well as the entire Tomb of Sargeras which was formally a Temple of Elune as well.
There were already connections deeply seeded into the story and setting. Demon Hunters and Demons and Gul'dan were literally just there for the sake of fan service. Legion itself could have been a literal Old God expansion that required the Pillars of Creation to fight
N'zoth. The entire Demon theme was only relevant to the starting expansion event, and to mid-tier and end-tier content. The rest of the expansion had very little actual Demon connections to it.