The Horde has proven that it doesn't need the Alliance to develop lore, we saw that with Garrosh's conflict with Thrall/Saurfang during and before Wrath, His ascendance and turbulence as Warchief, Thrall departure and journey in Cataclysm, the internal Horde struggle and now with the Revolution led by Vol'jin. Alliance contribution to that lore has been very minimal and rarely if ever pivotal.
Alliance on the other hand has never shined on it's own. When Varian returned/introduced he was immediately thrusted into a conflict with the Horde, we barely even saw him get acclimated with the leaders and workings of the Alliance. When Malfurion returned he was completely overshadowed by Thrall and Cenarius, he got next to no fanfare in Darnassus about his much awaited return or him possibly taking a leadership role in the Alliance. Even Anduin the future of the Alliance, his main storyline is peace with Horde.
The storyline of the human cities suffering from famine and chronic food shortages was dropped midway, the Gnomes reclaiming Gnomergon hasn't paid off, the Dwarves and their internal conflict never got the attention it warranted. Countless other storylines like Maiev feud with the Night Elf establishment, Dranei in general.
The question is does Blizz not think the Alliance can have an interesting enough storyline that doesn't involve the Horde somehow? and if so are they right?