The main reason is probably that they would have lost their TV deal, which is still tied to the Bay Area. Sacramento also made them an offer. And they probably want their first season in Vegas to be a true splash, in the brand new place, not the lo-fi version (no matter how nice the Summerlin park is).
I'm just not sure who's going to see them in Sacramento . I guess some locals are probably into the novelty but most fans in Oakland are probably just mad, and the AAA team there is the Giants. Also not the most exciting road trip for opposing fans other than the novelty. If I still lived in Oregon I guess I would have gotten in the car.