Answer: Because President Andrew Jackson refused to enforce the Supreme Court's ruling.
History/details:
The 1832 case, Worcester v. Georgia, ruled unconstitutional a Georgia law requiring non-Native Americans requiring a license from the state to be on Native American land. In responding to the case, the Supreme Court asserted that the federal government is the sole authority to deal with a Native American nation. From this Supreme Court assertion came the beginnings of tribal sovereignty within the United States for Native American nations -- that the US government would deal with them as domestic nations inside the United States.
The court case was named after Samuel Worcester, a Christian minister working among the Cherokee who was supportive of the Cherokee cause. To block the activity of a man like Rev. Worcester, the state of Georgia passed a law prohibiting white persons to live within the Cherokee Nation territory without permission from the Georgia state government. Worcester and other missionaries challenged this law, and the case rose to the level of a Supreme Court decision. The decision by the Supreme Court, written by Chief Justice Marshall, struck down the Georgia law and reprimanded Georgia for interfering in the affairs of the Cherokee Nation. Marshall wrote that Indian nations are "distinct, independent political communities retaining their original natural rights."
But President Andrew Jackson chose not to enforce the court's decision. He said at the time: "The decision of the Supreme Court has fell stillborn, and they find that it cannot coerce Georgia to yield to its mandate." He told the Cherokee that they would need to operate under the jurisdiction of the state of Georgia or else relocate. This was a step in the direction of what became known as the "Trail of Tears," when the Cherokee were removed from Georgia and moved to territory in Oklahoma.
