Here's why:

* Mollisols are known for their dark, fertile topsoil, rich in organic matter. This is a result of the region's history of grassland vegetation, which decomposes and contributes to soil fertility.

* Great Plains soils are specifically known for their dark, thick A horizon, which is the topsoil layer. This layer is rich in nutrients and provides a strong foundation for agriculture.

* Other soil layers are present beneath the Mollisol, but the topsoil is the most critical for plant growth and ecological function.

It's important to note that:

* Soil types can vary across the Great Plains due to factors like climate, topography, and past land use.

* While Mollisols are prevalent, other soil types, like Aridisols (found in drier areas) or Alfisols (found in areas with more rainfall) can also be present.