Because it's a substantial price increase for an incremental upgrade in quality and often a downgrade in convenience.
^^This! I have a Blu-Ray player (PS3), and I have maybe 15 discs bought since I got it, 3 years ago. The fact that I cannot bring them with me wherever I go and be assured that they will be watchable is a small factor, but a factor none the less. For me, it is the few discs I have gotten from Netflix with ads that I couldn't skip direct to menu, none of the menu buttons seemed to go past them. You have to hit next chapter/FF to go past them. That was enough for me to decide it was not worth paying for content that was going to force me to watch or deal with ads. In addition most DVDs are available for ~$15, whereas most Blu-Ray releases are ~$30 locally. Admitedly I don't have a 1080 display other than my monitor, but I do have a great 50" 720p TV, it has mattered a bit to me that I am not getting the full visual upgrade, but I can't afford another TV right now.
An Ada exception is when a routine gets in trouble and says 'Beam me up, Scotty'.