I think it might a range of factors.
Higher body fat levels lower testosterone. Body fat makes aromatase, and aromatase converts test to estrogen. Estrogen inhibits testosterone produciton.
There might be more environmental estrogens thess days due to chemicals and plastics etc.
Possibly the rise of feminism has made men lose their traditional roles so mentally don't feel as dominant and this can influence test levels. The mind can effect test levels, for example when you win at something test levels rise.