I believe the answer is: because John Locke believed that the government should only govern with consent of the people,
Taxation without representation refers to a system where people are being taxed without being able to choose a representative in the election. Because of his view, John locke view taxation without representation as an equivalent to a tyrannical government.
John Locke with his 'social contract' believed that the citizens of a country had the right to dissolve the government if it betrayed their rights so since Britain was taxing the 13 colonies at the same time it provided no representation ie was not democratic so since it stifled the colonists then they had a right to rebel.
