Why is is said that the delegates to the philadelphia convention ignored their instructions?

Constitutional Convention(May 1787)

Representatives from 12 states(Rhode Island did not attend) met in Philadelphia "for the sole and express purpose of revising the Articles of Confederation."Because amending the Articles required the unanimous consent of the states, the 55 delegates ignored their instructions and began writing the U.S. Constitution. Who were these 55 men?
-elite group of economic and political notables
-wealthy planters, lawyers, merchants
-college graduates with political experience
-coastal residents from urban cities

hope this helps.