why is it said that 'social responsibility is broader than legal responsibility of business ?

Dear student, 
Social responsibility is broader than legal responsibility. Social responsibility means that the business must undertake such activities that enhance the overall welfare of the society.  There must be overall development of the society. On the other hand, legal responsibility simply means working in accordance with the laws and other legal rules. 
In this way, social responsibility of business is a much wider concept than legal responsibility.