2. Globalization - Look around - there are almost always old warehouses either for food, lumber, or a small manufacturing plant that are run down - usually a few blocks away from downtown. These places one by one couldn't compete manufacturing cheaper goods from china or elsewhere
3. Walmartization - big box stores come in from out of town, suck up all the economic activity, pay people nothing, and ship all of the profits to HQ. Then the walmart closes, and there is nothing left.
There has never truly been anything in most of these towns. A good number had rail, canal, or other trasnportation connections, which made them viable (yet still small) at one time. Now most are really dependent on the next biggest "city" outside of the farming communities that bring them together. I Think the reason they get worse is because the hub cities - places like Kokomo, Marion, Muncie, and to a lesser degree Lafayette - have suffered. Manufacturing jobs that held these together no longer do that. Even outside of the "good-paying" factory jobs, it used to be OK to drive to the nearest city for a basic job. That's no longer the case (even though fuel prices have improved). And you wind up feeling like you are stuck.
Cars reshaped the landscape as stores / restaurants / movie theaters etc could draw from a large enough population to support themselves. However, farm automation drastically reduced the number of jobs causing young people to move out. Eventually you can't really support much, but the travel distance allows for some shops in these nano towns.
Remember, 80 acres used to be a viable small farm. Now 800 acres are generally just a hobby unless your raising livestock and buying a lot of feed.
https://en.wikipedia.org/wiki/Rust_Belt
“Before World War II, the cities in the Rust Belt region were among the largest in the United States. However, by the twentieth century's end their population had fallen the most in the country.”
Knowing nothing else about the particulars of why this was the case, it’s easy to see that this change would have drastic consequences for these areas.
> Like many rural towns, Decatur once fed itself and shared its bounty with the rest of America. At the turn of the 20th century, it had a tomato cannery and exported peaches, apples, strawberries, and beans by the boxcar from a downtown train depot. A population of 245 in 1915 supported two grocery stores and four general stores. But in the 1930s, blight outbreaks and insect invasions largely wiped out local orchards. The depot is now a museum where antique bushel baskets and poplar harvest crates hang from the ceiling.
To New York and London of the 1800s, the Midwest (especially) looked a lot like China did to us 20 years ago -- a massive pool of resources that could be exploited using the new technology.
Anyways, this town was founded as a railroad hub in the 1880's. Both a freight hub taking farmers' harvests and delivering goods to the farmers, and a passenger hub transporting people from the surrounding counties into the nearest big cities. The town also provided necessary services at a time when traveling more than 15-20 miles was a hardship. There was a grocery store, general store, cinema, funeral home, pharmacy, bars, couple of hotels, hardware store, and so on. Was a central gathering place for the farmers, and the people who provided services for the farmers.
Many of those businesses are long gone now. The town is still vital for farming because of its mill & grain elevator, and the hardware store still does great business. But the rest is gone. Even the railroad moved away. There are no tracks left in town and the main route is 20-30 miles away. The town survives because of inertia. And because some people are willing to commute an hour to their work in exchange for having a house in the country, and a school with small class sizes for their kids.
My parents grew up on either side of a small village that now has a dollar store as one of 4 or 5 shops. It started when the railroad came in to log and became a "potato town" where farmers brought their potatoes to the railroad.
There's a museum in the old potato warehouses.