Simple rules just cannot address their issues and the nature of their business. These rules have been adjusted and modified significantly as a result of comments from bankers and other interested parties. Sometimes those comments have led to simpler rules.
But, more often, they have led to even more complex rules because each complex bank to which they apply operates somewhat differently from other banks. Thus the rules have been modified  to address important and meaningful differences in risk. Simple rules too often become straightjackets; flexibility requires more complex rules.