The problem with the original question - "Why are Blizzard spending less time on Alliance content" - is that its impossible to prove. No-one at Blizzard will candidly admit that half of their dichotomy routine is consistently underdeveloped, and many of the metrics used to score how much 'work' was put into a faction winds up being subjective. So the question becomes more of, "Why do people who play Alliance feel as if their content is subpar?" I won't hesitate to say I'm a longtime Alliance player, and I feel personally that Alliance content tends to be more dry than Horde content.
My analysis is that the dissatisfaction comes mostly from how the faction stories are handled. A great many of the Alliance stories are based on things they have lost, whereas Horde stories are more about the things they have gained. This principle ranges from territorial (Alliance loses Southshore, the Horde gains control over Hillsbrad), to racial (The Worgen lose Gilneas, the Goblins gain Ashzara), to cinematic (While both factions lost their leaders at the Broken Shore, the Alliance story focuses on the loss of their king, whereas the Horde story focuses on the rise of Warchief Sylvanas). It leads to Alliance players feeling like they're never truly winning anything, the best they can hope for being some high-road return to the status quo.