Because it's one of the first 10 as originally (or close enough) set down by the authors of the Constitution, consituting the Bill Of Rights.

http://en.wikipedia.org/wiki/United_States_Bill_of_Rights

>
>Here's another question that bothers me a lot. Let's take for example, the 18th amendment to the constitution. That one abolished liquor from the U.S. and it was ratified on January 16, 1919. It was subsequently repealed by the 21st Amendement ratified on December 5, 1933 (just in time for xmas apparently). Ignoring, for themoment, the original stupidity of the 18th amendment, the point I want to make is simply that amendements can be made and they can be changed, or even repealed by further amendments.
>
>For some reason though, everybody talks about the 2nd amendement as though it is somehow sacrosanct and, well, different from all the other amendements in that it may never be questioned, changed, repealed, or even discussed except in some sort of veneration and awe.
>
>I know people like their guns, but what is it about that amendment that makes it so holy compared with other amendments such that it may never be questioned?