One, because to be honest up-sampled DVDs look pretty good.
Two, most movies are now shot in a style that looks like someone let a cat piss on the celluloid. Seriously, who the fuck wants to pop in the Battlestar Galactica Blu-Ray and see film grain and shitty lighting in hi-def!?
Three, for the price point, Blu-Ray doesn't deliver enough value except for the rare really well-shot movie.
Four, digital downloads. All things being equal, anything on disc is slightly antiquated. I rarely buy a disc of anything anymore.
Every nonzero finite dimensional inner product space has an orthonormal basis. It makes sense, when you don't think about it.