I'm going to guess DVD is still popular because of the elderly, children, and families with lower income. DVD is the defacto common standard playback device and I think every household has some type of tech that will play one. The elderly peeps that I know cannot see the difference between SD or HD, or not enough to care anyway. H*ll, my mother would still own a VCR and be buying tapes at flea markets if she had her way. And for kids... parents will pick the lowest price option for kids shows/cartoons. Have you been into a discount store like Big Lots or Dollar General? They have huge selections of bargain movies and they are all DVDs.
edit : I forgot another target audience... people who live in areas where you can't get decent bandwidth to stream, or it costs too much money. My bro-in-law has a place in the mountains and his internet is 1mb DSL, and most phone service doesn't work there. He has a pretty decent physical disc selection to keep the wife and kids entertained. Some are DVD's, kids don't really care just as long as they can get their cartoon fix.
VLC can play blurays that do not include crazy levels of javascript menus and such. Also MakeMKV can rip your BR down to an MKV that will play on just about anything. If you feel like spending money, Anydvd and others like that.
