Because the other 1000 innings of Wood's career show that he's a lot better, and his peripherals are better this year too. He's also 4 months younger than Rusin and has better stuff. but other than that, can't see any reason.
Is there really any reason to give Volstad another start? Wood looked pretty decent against the Dodgers. I know his AAA stats are pretty crappy, but at least he has the PCL excuse.