Why are professional people also suffering?

Professionals suffered because their customers (the farmers) suffered. In many cases, farmers in need of services paid with what they had available... their crops.

Atticus said professional people were poor because the farmers were poor.

As the Cunninghams had no money to pay a lawyer, they simply paid us with what they had. “Did you know,” said Atticus, “that Dr. Reynolds works the same way? He charges some folks a bushel of potatoes for delivery of a baby.