Jaitea said:

Why couldn't R2 switch the Tractor Beam off from the Hanger control room, that would have saved a bit of bother

J

As Obi Wan has to physically turn some switches, it's probably not designed to be remotely fiddled with by droids. I doubt Artoo could switch it off any more than he could cause a reactor overload and blow up the station.

As it's just one power terminal that's deactivated, I'm guessing the overall system appears "on", but is weakened, or would take time to find the problem before the Falcon makes a clean getaway.