Diamond is used in some of the higher end thermal pastes and they do perform marginally better than alternatives, but the problem is that they tend to cause micro-abrasions caused by the high-speed vibrations ICs do. This isn't too big of a deal when you're mounting a cooling block to an IHS surface, but if you're doing direct-to-die cooling (which many hardcore enthusiasts do) it can spell disaster.
Well, boron nitride should be almost as hard and abrasive as diamond. I'm guessing it's just the law of diminishing returns that works against diamond here.
