There are many factors.
Big game tends to need big habitat. So in some places, deer are being pushed out by housing development. This leads to fewer tags and more "no hunting" areas where the deer still are.
Pheasant populations are down in many places because of "clean farming" practices that doesn't leave habitat between fields.
But in other places, there are "too many" deer and pheasants everywhere. There are now some prime deer hunting areas where, a few decades ago, there were few if any deer.
We are wealthier than we once were. Few of us will spend our entire lives hunting with a single pump gun and a rusty old rifle. A generation or two ago, this was common. But that wealth also buys us houses and moves us into cities and metro areas. Few of us earn our living in agriculture. Again, a few generations back, most of us did. This means that fewer of us hunt deer as "targets of opportunity" in our fields and surrounding areas. Given the extra effort and expense that most of us have to put out in order to hunt vs. our grandfathers, I'd say that hunting is thriving.