Because it denies the supremacy of federal law as expressed in the Supremacy Clause.
Walt
Your interpretaion of that clause is definitey counter to several other clauses in that same Constitution.
Clauses I.2.3, I.9.1, IV.1, IV.3.2
And of course Article VII The Ratification of the Conventions of nine States, shall be sufficient for the Establishment of this Constitution between the States so ratifying the Same.
Note it says between the states. One would think that 9 states would also have the right to abbrogate that Constitution between them.