There is a big difference between enacting a new amendment (18th..your reference) and repealing that amendment later (21st). The 2nd Amendment is part of our Bill of Rights. That makes it an integral part of what makes our country unique. These first ten amendments are rights that we the people are guaranteed. At some point there may be severe restrictions as to where we the people can exorcise those rights, but in order for our republic to survive, those rights must remain sacrosanct. If not, we will no long remain a United States.