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.
If in any problem you find yourself doing an immense amount of work, the answer can be obtained by simple inspection.