Because the Amendment was not properly ratified. That's "all" it takes to make it UNconstitutional.
- SL
I hate to be nit picky, but that's not entirely true.
Article V
"The Congress, whenever two thirds of both Houses shall deem it necessary, shall propose Amendments to this Constitution, or, on the Application of the Legislatures of two thirds of the several States, shall call a Convention for proposing Amendments, which, in either Case,
shall be valid to all Intents and Purposes, as part of this Constitution, when ratified by the Legislatures of three fourths of the several States, or by Conventions in three fourths thereof, as the one or the other Mode of Ratification may be proposed by the Congress; Provided that no Amendment which may be made prior to the Year One thousand eight hundred and eight shall in any Manner affect the first and fourth Clauses in the Ninth Section of the first Article; and that no State, without its Consent, shall be deprived of its equal Suffrage in the Senate."
So... if an amendment is not ratified, it is not for all intent and purposes a part of the constitution. Unconstitutional means contrary to the constitution, it does not mean that something is simply not a part of the constitution.
This may seem like a small distinction, however because all but one of the amendments were ratified by state legislatures, those states that ratified have agreed and made it valid, as it is then a state law. Those states that did not ratify it may have a case to make, however none have successfully made that case.