They are not all in PA, there is a good supply in the mid-West. They made about a million of them through the years so I would expect them to have a good supply.
Now as to why in PA, who knows but that's a relatively rural state, along with the mid-West and this tends to be the heart of the JD Garden Tractor market. I would say that market goes from the central Atlantic up to the North East and then out to the Mid-West. I am guessing lots of folks like to garden or have big yards, although these things were not cheap back then so maybe the per capita income of the rust belt states is/was higher and it was easier to sell in this region, then in the South back then. Obviously there probably wasn't much of a market for these in the SW US.
This question comes up every now and again. I don't think anyone has really answered to any satisfaction. It would take someone who worked at the corporate JD offices back then to maybe explain their market.
I have lived in the South my whole life and the tractors of choice back in the day were smaller utility tractors not garden tractors....sure you can find a few but not many.
Ford 8n's, cub lo boys, small Farmalls, row crops and utility tractors are everywhere down here most commonly mated with a 4 or 5 foot belly mower or 3 point bush hog.
Garden tractors never really did catch on with the farming community down here, usually the way we got one is if the dealer would throw it in for free as a deal for buying larger tractors.
We purchased a fleet of 4020's in the early 1970's for the farm and the dealer gave us a 110 for free in the deal, I still have "parts" of that 110 to this day.
I think the northeast has so many due to the landscape (smaller farms) and weather....I see so many up there used for snow blowing.
I found a 140 the other day it had been used at a small airport for about 25 years as a "tug" to move the small planes around.