In fact, across the entire PC market there's not a lot of wholehearted support for Blu-ray or any major high definition formats. It's not that DVDs are "good enough" -- HDTVs are selling by the truckloads -- it's more that consumers, apparently, just don't want to settle on another format. And that may be the key to this whole thing -- Apple has a vested interest in selling content, and implementing some other content producer's format into their machines will take away from their best HD content channel yet: iTunes.
And customers, happy to not have to buy yet another permanent format of their favorite movies, may be satisfied with having no next-generation disc format. I, like many users, have already watched tons of HD video without ever having bought a Blu-ray disc. If Apple doesn't need the drives to deliver the same quality content, why should they bother?