CD is good enough -more than enough for the average consumer- and CD players are widespread, while BluRay is so far limited to video players, computers, and high end audio gear (which probably also offers video playback, because it'd be a small additional expense when producing a $1000 device
). There's also a whole lot of equipment on the manufacturing side that would need upgrading to produce BluRay discs, and the media is cheaper as well. So the big guys have little motivation to push it into the mainstream, and it will remain a niche product, just as SACD did (and hopefully not die of like DVD-A).
EDIT: I should clarify that, while newer formats like BluRay are better, CD's are already fairly good; 16bits if enough to cover the entire frequency range that humans can hear (a little more than, actually), and most recordings these days only use a fraction of the dynamic range available to them (in order to make the album louder, it gets squished down and squeezed into the absolute top of the range, which can lead to clipping and other issues). So there's a lot of potential gains to be had simply from making better use of what we've got.
Earlier Mogwai material makes full use of dynamic range (I gather their more recent stuff has diverged on that front), and I've found Mad Season's
Above (their only release...) to be an excellent example of a well-mastered CD (if there's been a remaster, you'll almost certainly want to avoid that
). I assume classical music is usually mastered to a high standard, as your typical listener would be wanting maximal clarity and to hear as much detail as possible (and would be rather upset if the quietest parts of the composition -which are sometimes barely a whisper- were artificially altered to be nearly the same as the loudest bits).