Something tells me that, besides the weather, the style of play in the Pac-12 probably matches up better with what he can and likes to do better than the Big-10. Of course, he has to live in LA (ugh). I know that many of the posters, including me, had serious concerns about his ability to defend and basically hold up under the Big 10 grind. I don't think that will be as much of a concern for him at USC.
It's also possible there are specific academic concerns or goals, that none of us know about, that may have made him take a second look.