It's because the Tau have the lolrailguns that obliterate anything that they hit. The only thing that would be a major threat for the Tau is the Necrons and Orks. Let's face it, the Imperium will only make the Orks stronger until the Orks start an epic WAAAGH! that sweeps through the entire galaxy and curbstomps the Tyranids, Necrons, and the Imperium. The Orks only get stronger by fighting the Tyranids.