Something else occured that makes me lean toward the generational thing: Back in the early 80s there was a backlask against D&D by the Christian right.
I don't know how much impact it had in other parts of the country, but here in the south it was fairly profound.
There were extreme cases -- groups burning D&D books -- and other, less drastic episodes that I remember vividly. I had friends at school who simply quit playing the game, and knew of parents who wouldn't allow their kids to play the game.
These incidents may have taken a number of copies of OD&D and
1E material out of circulation, and had an impact on the availability of material now.
Just a thought ...
Keith