Because that is 2 more times your faction was the center of the story then the Alliance.
Just think about it. Ever since Cata the story is entirely driven by the Horde's terrible choice in leaders. Garrosh has kept us busy for 3 expansions. MoP, WoD and Legion are all results of his decisions and his decisions alone, now we have Sylvanas and she has already been the center of the story in BFA, now continuing in SLs. Probably whatever she is doing will effect the next expansion too.
Being the Alliance many times feels like being ineffective extras in a series about the Horde.