"Part" of what contributed to a lot more gobbling then, was the ratio of adult hens to adult males. Since there had previously been no hunting, it was much closer to 50-50, causing more "competition" among the males. As hunters knocked down the adult male population relative to the adult females (talking about 2-yr-old & older birds here), there became progressively less gobbling.
Over the last few years in most areas of TN, I believe we've been progressively killing a higher percentage of the living males 2 yrs & older, this being a significant reason for less gobbling. We can turn this around, but it may take at least 2 years. Some of this has been camouflaged by the flocks expanding into areas of previously little or no turkey hunting.
Another reason for less gobbling is one we probably cannot turn around, meaning we may never again experience the amount of gobbling we did in TN a few decades ago. This reason for less gobbling is evolution, and greater survival of those males genetically programmed to gobble less.
There has also been an evolution in predators such as bobcats, coyotes, and the largest hawks & eagles. They have "evolved" to go to the sound of a gobbling bird. Those individual males which gobble more, they're more likely to get eaten by a predator, including human predators.
Experienced turkey hunters often observe that really old Toms often don't gobble on the ground, and come in to a call "silent", and often taking hours to move a few hundred yards to your calling. I'm now seeing many 3-yr-old birds being almost totally silent after they hit the ground, whereas over 20 years ago, most would have been very vocal. Part of this is evolution, part of it is just older Toms are wiser.