"As to removal by recall, the United States Constitution does not provide for nor authorize the recall of United States officers such as Senators, Representatives, or the President or Vice President, and thus no Member of Congress has ever been recalled in the history of the United States. The recall of Members was considered during the time of the drafting of the federal Constitution in 1787, but no such provisions were included in the final version sent to the states for ratification, and the specific drafting and ratifying debates indicate an express understanding of the framers and ratifiers that no right or power to recall a Senator or Representative in Congress exists under the Constitution."
In 1787 they weren’t elected by popular vote.