I think there are several reasons. Demographically, I'd think that many 50 plussers are retired or reaching retirement and not all have the income at their disposal to buy the rides or equipment they'd like to own. Others support multiple families or have other commitments that don't allow the spending.
Then there's the group that doesn't necessarily feel that newer is better. I personally suscribe to this group. Under 20 pound bikes go back to the 1800's. Track speeds of over 40 mph go back to the early 1900's. Shoot, they were riding high-wheelers around the world in the 1800's. Marketing often drives bicycle development, not necessity. In sales, you need newer flashy--bling.
Too, there is good equipment available used--at garage sales and on Craigslist--even at some bike shops for considerably less than new. People trade up, people get in over their heads and liquidate, people buy and don't ride. Not everything is junk.
Then there's a group that basically feel that it's none of your bleeping business
They ride because they want to ride. A modern carbon fiber featherweight isn't going to make them faster, stronger, better looking or more attractive to the opposite (or same) sex. Many can afford anything they want, they just don't want what everyone else has. They want to ride.
Armstrong said it best when he named the book: it isn't about the bike. It's about the men and women who ride them. That's what I love about this forum--it's more about people who ride bikes, not the bikes themselves.