Because Blues is not a music which preaches destructive behavior and anti-social gangsterism. It is not part of the culture which promotes rebellion against the "Man" the "System" etc. The Blues is a music appreciated by the mature and does not feature the Loud, heavy beat and sadistic lyrics the young want. Nor is it really a music to dance to. It is a music of reflection and introspection to a surprising degree. And tells stories.
And it is like asking why young whites don't like Frank Sinatra in being a generational thing.
Well, that's just sad.
As a young person myself, I like blues, and jazz, and I love music that is introspective/reflexive, and thought-provoking. Actually, movie soundtracks are my fav. I don't like angry, loud, rebellion, etc. Honestly, I'd take Sinatra (or Mel Torme) over heavy metal any day.
Then again, I'm a freak.