Well, there are a couple of reasons, but the big one has to do with grass. Let's jump back a few millions of years though, and we will find that much of what is now the midwest was a shallow sea. This is going to be helpful, because the bottom of that sea will be fairly flat and level. Fast forward to more recent history, and we end up with wide, flat plains. The plants that have evolved to inhabit these plains will be adapted in a number of special ways. First, they will be perennials. This just means that the exposed part of the plant above the surface will die back in the cold winter months, but the root system will survive. In the spring, it regrows. This enables it to survive cold winters. It also gives it an advantage during dry summer months. If lightning starts a fire, the top exposed part of the plant can burn up, but the roots below ground will survive.
So why do we care? Well, every year, the roots of these plants (mainly grasses) grow a little deeper, a little longer, and a little wider. The ashes and dead leaves at the surface from winter and fires will be trapped in the dense growth above. Every year, some of the root system will die as well. As a result, these grasses are constantly pulling carbon dioxide out of the air, turning it into plant material, and then turning a little bit of that plant material into dead plant bits. When bacteria and funguses and other organisms break that stuff down through decomposition, it turns into soil. Rich soil, filled with important nutrients necessary to grow more plants.
So, basically, the soil in the plains is so good for growing plants because the plants that used to be here were great at making rich soil. In fact, grasslands can build about a qurter of an inch of new soil a year. Not bad!
On the down side, back in the 1800's, John Deere popularized a plow with a metal blade that could break up the dense root systems of the grasses. This allowed us to plow up the plains and replace them with farms, which benefited from the rich soil the grasslands had built. On the downside, our farms DON'T build up soil. In fact, a combination of poor land use, poor farming practices and a lack of erosion control has meant that the rich soil of the midwest has slowly been getting used up: depleted of nutrients or washed away. In some places, where there was once over ten feet of rich topsoil, there is now only a foot remaining.
This is one of the reasons some people are trying to grow food crops that are also perennials like grasses with deep root systems. That way, the plants would grow food for us to eat as well as build up new soil.