It's too complex of a phenomenon to reduce to one, singular explanation.

However, increasingly sedentary lifestyles are definitely part of the problem. Jobs encourage people to sit motionless, of course, but most of our leisure time has been replaced by other motionless activities.

Global obesity rates have also been rising steadily while testosterone levels have trended downward. Obesity is well known to reduce testosterone levels.

There might be additional answers in environmental toxicities, but it's a mistake to ignore the elephant in the room: Lifestyle is a problem.