The answer is: the stamp act directly affected the colonists; taxes prior to the stamp act were indirect taxes, paid only by merchants.
According to the stamp act, The British government is eligible for some payment every time the people of the colonies are using printed materials in their day to day activities.
These materials are used and exchanged everyday by almost all people and the usage happen frequently. Because of this, the majority of people felt very dissatisfied with this law and cause boycotts on British's product to enter their territory