It's why the Founders didn't put it in the Constitution or even use it during the American Revolutionary War. It first came into play during the War Between the States, as I learned from this article.

They also didn't include an income tax when the U.S.A. was created, which is another form of slavery. The War Between the States brought about the end of slavery for Black Americans, but the income taxes that were first introduced then, as well as conscription, just extended slavery to all Americans...