Why did the colonists resent the tax on tea

Unfortunately for Townshend, the Stamp Act had aroused colonial resentment to all new taxes, whether levied on imports or on the colonists directly. In 1770, Parliament repealed all of the Townshend Act duties except for the one on tea, which was retained as a symbol of Parliament's power over the colonies.

The colonists had no representation in the Britain government (no say on taxes).

Hence the phrase "No Taxation without Representation".