IIRC, a paraphrasing of the 'Into the Storm' explanation for Orks un-corruptability was something along the lines of...
"...Orks seem like they should be natural followers of Khorne, but the typical Orkish mindset thinks, if he <Khorne, or some other greater Khornite Daemon> isn't going to show up here and now to beat our current war-boss in a fight, why should we worship/follow him, instead of the afore-mentioned war-boss who is here and is leading us in battle and is beating us up his-very-own-self? "
which just made me love Orks even more!