"Why" is rarely the right question for fashions. "How" works better: the party was a little cool, so cool people were more likely to associate with it, so it became cooler, so cool people were even more likely to associate with it.

Now, it's just visceral. A rally against the double-taxation of capital gains and dividends just doesn't have the cachet of a rally against hunger. This despite the fact that economic growth (the kind you get when taxes aren't excessive) has done a lot more to stop hunger than any rally or government program ever did.

Doesn't coolness reach critical mass at some point, where the really cool people drop out and do something different? Which is why fashion changes so much?

Also, why not just dress the rally against boring sounding, but important things, in trendier terms?