Here's why:

* Deciduous forests are characterized by trees that lose their leaves seasonally. This leads to a high input of organic matter into the soil.

* Alfisols are known for having a good balance of nutrients and a well-developed structure. They are also typically found in areas with moderate rainfall and temperatures.

While Alfisols are the dominant soil type, you might also find other soil types in the eastern US deciduous forest depending on specific locations and conditions. Some examples include:

* Ultisols: In areas with higher rainfall and leaching of nutrients.

* Inceptisols: In younger, less developed soils.

It's important to remember that soil types can vary within a region, and these are just general categories.