I don't know. But I suspect it's some kind of emotionalism. A few weeks back we saw Oprah in full-throttle rave about Denmark's collectivist system. But it's just not rational to believe that what's satisfactory to 9 million is going to work the same for 300+ million, particularly given that we're talking about the most diverse nation on earth.
They don't think about what they'd have to give up in exchange for achieving their goals. They don't think about what America would look like
without the protections afforded by the U.S. Constitution. They don't think about the fact that what they're insistent upon is "top-down" government, not the "bottom-up" federalist model we were provided by our founders... or what the effects upon our diverse culture would be.
The depredations upon federalism have already made our nation nearly unrecognizable from the original model.. and what we see is a system where the scale is so large that there's no room for mistakes. Say a State ***** something up... there are 49 others which haven't. But this one-size-fits-all method means we're ALL in the soup when something goes wrong.
This is going to piss a bunch of people off... but I truly believe that it's a small mind, an
emotional nature, which craves uniformity. The left makes claim to the appreciation of diversity, but there's a world of difference between words and action. The insistence upon a cookie-cutter version of "diversity" doesn't do justice to federalism. It's NOT okay by them for California to do it one way and Texas to do it another. They don't see the value in REAL diversity. They don't see the risks we face when we put it asunder. Their version is an emotional response to social issues, an arbitrary designation of "fairness".
The use of the Commerce Clause as an all-purpose tool to manage our economy as a whole has left no room for human error. A butterfly can't flap it's wings. We see that clearly in the sub-prime meltdown and its effect on the credit crisis. Without macro-management from Congress, maybe only a handful of States might have stepped in that particular cow patty, preserving the economies of the rest.
The world is a chaotic place. And I think sometimes there's a natural human impulse which demands we attempt to bring order to it, but I also think that it's a base impulse which should be ignored for the sake of higher thinking. Let us not forget that Islamic Extemists also demand "uniformity".

Not that I'm saying leftists are in that particular blood-thirsty category... but the underlying emotional insecurity, the need to control the environment and everything within it, is pretty much the same.
What's worse, is that by seeking this totalitarian-type control, we lose what little control we have. Our votes are powerful at the local level, affording us a small amount of democratic power in ordering our local communities. But at the national level, our vote is so diluted as to be nearly worthless to us in our ability to affect our own microcosm.