Squirrel populations vary from year to year with the feed crop. Poor seed crop, fewer squirrels. Lots of trees that produce large seeds, the seed crop differences may not show much. A few years ago we had them under the house, in the garage, in the barn. Live trapped and drowned them. Next year, no seed crop, no squirrel problem.
When we moved into our house in the woods in Montana we had a lot of them, but after 3 years we hardly saw any more. May have been the seed crop or may have been our cat, or a little of both.