Well, there weren't specific guarantees in the U.S. Constitution itself, so the Founders thought it best to add it as an amendment.
It was very wise of them, imo. This way, it is almost impossible for people to get rid of them, because amending the Constitution is almost impossible to do.
Republics always turn into democracies, and democracies can often degenerate into autocracies if enough safeguards aren't built in.