The aesthetic, the quality, the must-haves, and a bit of sentimentality. These bikes are what we're passionately familiar with when we were young, and now older, can afford and still appreciate the old, and that the latest isn't always the greatest.
I have nothing against today's stuff. I just don't personally find a need to upgrade to it. I still get plenty of buzz from my older toys.
All too often the younger set don't care for older tech as they have no history or respect for it.