I know some people who don't wave since they don't know the person, why wave they say..
I personally will wave, if waved to, i usually don't go outta my way to get a wave out there just to be snubbed.
But like the other guys said, maybe he was new, or just not in the mood. Hey, he may have waved if going the other way moving, but since sitting at a light, he didn't.