Answer:
It's too cold for plants to grow.
Step-by-step explanation:
The average temperature within the great plains is less that 40 Degree Fahrenheit. Meanwhile, the average temperature for Agriculture would be ideal at 77 - 88 Decree Fahrenheit.
Because this, many type of crops that provide high provide unable to grow properly in Great Plains. So, the people there chose to do another economic activity to fulfill their basic needs (such as herding and collecting timbers)