Why did the Ithryn Luin go?

[the guy who be short]

Perhaps Alatar and Pallando played their part too. Perhaps their influence in the east denied Sauron of at least some of the Easterling armies that, without their participation, would have been available to him. Perhaps those extra forces, had he had them, would have been deployed to devastating effect. Pure speculation, I know, but I like to think that the Blue Wizards, or at least one of them, did their duty.

Not speculation at all. I remember reading somewhere, and I think it may have been by Tolkien himself. It went something like: The purpose of the Blue wizards was to teach and preach to the Easterlings, who otherwise would have come with great force and utterly crushed the West.

I, for one, believe that they did succeed in this, but that they were rather impassive in actually fighting Sauron, which is why they "failed".