I had gotten that figure from a 2016 report put out by the JAMA.
Interestingly, even though the eligible men increased, the percentage of TRT prescriptions increased more dramatically. So, more men were coming in with symptoms, but even more were still being diagnosed and treated.
I do think, like lowcountry said, it's a combination of factors - lifestyle, environmental, and nutritional - there's no single pinpoint as a trigger. Xenoestrogens like BPA are devastating to T levels, but also, I think there's been snowballing anecdotal and empirical evidence over the years connecting hormonal health to overall health, so doctors are (slowly and steadily) getting more on board with prescribing TRT.