I'm almost certain the reason is licenses - DVDs licenses have long expired and back when they were in effect DVDs were the mainstream option, so lots of devices and machines around for them when licenses were needed and that gave momentum to DVDs. Blurays on the other hand still require licensing and they never became as mainstream as DVDs.

I can find a bunch of small businesses pressing CDs and DVDs but they rarely provide a Bluray option. I have a feeling that even after Bluray patents expire, DVDs will still outlast Blurays :-P.