Lots of guesses but none that I personally believe would have had that much of an actual, immediate, and ongoing impact for so many years now???
My guess is because in most locales, it hasn't been any one thing, but more a little of everything at the same time being more detrimental.
In certain areas, there have been certain single factors that have been documented as more significant. For example, in some areas it has been bear predation as the #1 factor, but bears are not in Western Middle & West TN. But, even that bear predation in those areas may have been augmented by mature forests lacking adequate fawning cover.
I'm just "guessing", too, but I believe in much of TN there has been significant increase in fawn predation by dogs (yes, household pets). This is mainly the result of more houses popping up in places previously void, and many of these people have dogs they let run loose at least some of the time. I've observed people letting their big dogs out @ 10pm, then getting trail cam pics of those dogs 1/2 mile away less than 10 minutes later.
As a "for example", at some point in the past, homes
may have averaged 1/2 mile apart on the county roads. Maybe today that distance is 1/4 mile. Just saying, a lot more houses where there were none a decade or two ago. And a ton more roaming dogs from those houses.
Turkey populations come to mind over exact same timeframe???
Yep. Many the same factors that can harm fawn recruitment also harm poult recruitment.
But much worse, the turkeys have a lot more going against them,
in particular raptor predation, its evolution in targeting turkeys more, and a ton more raptors than in times past.