Ah, the heart of the issue. Although these "conservatives" claim to be for decentralization of power, they only follow through when it helps their cause.
If the design of the republic was honored, and we truly had 50 laboratories of democracy, much of what they stand for would be threatened. A successful libertarian state would undermine much of their chicken little "the sky is falling" claims. Most importantly, it would set example and precedent for other states.
This deeply threatens their designs for Empire. Imagine a country where they couldn't < gasp > control other people! These people are control hungry power junkies. Even the worst junkies addiction pales in comparison to their hopeless addiction to power.
Much, if not all of their goals would be undermined by a small federal government. The 10th Amendment poses more of a threat to them than any of our foreign enemies. They will fight state's rights tooth and nail. And if necessary, they'll send another Sherman to burn it to the ground.