Why Keystone? Roberts mentions it, but I'll emphasis again one big reason which I think was often lost: because it was Obama's call. Very often it's ludicrously easy to spread the "blame" around for things in Washington. If everyone is to blame, no one is, and there isn't a target for that kind of activism. It was possible to say, clearly, who would be responsible for it one way or another. That isn't the only reason, of course, so go read the whole thing, but it was an important one.