Nobody really knows why the London Tube is getting less and less crowded

The population is aging. People less opportunity to work for wage that can support the price especially if its anything like the Netherlands, paying to much to go from a place you don't want to be to a place you don't want to be either sucks. Even with traffic jams public transport can't compete unless you like torture.

Might just be the people who were key riders just aged or fell out of income bracket that can use it.

I ride a motorcycle year round instead of using 2 different public transport vehicles and walking that makes my journey 55 minutes instead 20 at a good time of day.

Costs me nearly nothing in fuel, no parking cost while the public transport is up to 10 euros a day.