Dad said it was because everyone pulled out the fence rows and plowed from horizon to horizon. Then they burned out the ditches. Loss of habitat.
Dad hit the nail on the head here.
This is only part of the equation, though. Yes, the expansion of Midwestern farms is part of it, but in a broader sense, it's the "unification" of foliage type by region.
For instance, here in the Midwest, it's as your dad described. In the Northeast and most of the East, really, you've got vast swaths of forest which are not allowed to be logged and where, if a fire is detected, is swiftly put out. Forests all throughout the country are treated this way. This leads to an aged, overly dense forest where there is marginal undergrowth for small game and food - pushing said small game and food-seeking animals either closer towards extinction or towards civilization.
The same things have happened in places like New York State, where as little as 30 years ago hunting for grouse, partridge, and pheasant was a commonly enjoyed pasttime by many residents. Today, where there miles of multiple small farms - some corn, some wheat, some orchards - interspersed with tree lines and fields left to seed, there are new 'developments' of town houses and appartments, wooded areas which are not allowed to be "deforested", and farms converted over to single-family homes for urbanites with large yards.
You can compensate for this a little bit by having game production areas funded by the state, and the US has more of these than any other country to the best of my knowledge. If you've been to many, you'll notice how most of them have very diverse habitat: some reeds, some tall grass, some cut grass, some seeded corn, some trees, some brush, some young growth and some old - etcetera. Some have swamps and various other 'produced' land features which would not have been there naturally, but have been placed there due to the fact that it's a game production area. Such diverse habitats are necessary for a diverse, thriving ecosystem.
Even the American Indians would routinely burn down swaths of forest - as a matter of practice - in order to help encourage wildlife growth and diversity. Burn down a section of forest and you end up with a young area of forrest where generations of deer, pheasants, and other herbavores can eat heartily on low-lying bushes, pine buds, sapplings, berries, and moss. Let that forest grow into a towering canopy - like the Redwood Forest - and you will have precious few wild animals roaming about.