I used to point out that there wasn't a single reason. Dick Cheney's reason was not Tom Friedman's reason, something Tom understood without managing to take the next leap to understand that Dick's reasons were incompatible with his.
But for most our country's prominent dipshit centrists, the reason was, basically, to prove they weren't stupid pussies like those dirty hippies.
Can sell them on anything by telling them The Left opposes it.