There are a number of things that contribute to low small game numbers here.......
Back in the "Golden Days" of PA Pheasant and rabbit hunting it just happened to coincide with all time highs in fur prices. Anyone who could set a trap in PA was trapping fox.
As a kid I heard stories of hawks shot on sight, and pole traps and other nasty ways farmers dealt with them.
There were simply less predators, the birds(pheasants) were a heartier stock, there were MANY MANY more farms and habitat was better overall......these days I don't know of many people who find it worth their time to trap, hawks and other raptors are now protected at all costs, add to that mix feral cats and an increasing coyote population and you have a very efficient killing force prowling what cover is left endlessly.
Too many IMO......that's one reason I have no problem hunting fox every chance I get. They are great sport and make for beautiful mounts if you don't wish to sell the fur.

Small game (namely Pheasants and rabbits) will NEVER EVER reach levels seen in past years.

You want a prime example?.......a buddy of mine joined on of those Pheasant Hunting/shooting clubs a few years back. Where you buy so many birds and stock them yourselves and then come back later in the day to hunt them.
He told me of one day where he put out 10 birds, and came back 4 hrs later.......in 4hrs 3 were killed by hawks, and another by a feral cat in the middle of the day. (He chased the cat off the one bird)
So that is 4 out of 10 birds killed by predators in 4hrs!!! Now try to picture the impact that killing force has on the habitat day in and day out?