I can't find any reasons why but, in general, I suspect that Oxford prefer a good, consistent, 'all rounder'. Even the fact they offer more joint honours courses than Cambridge supports this. I know they still mostly care about you being great at your chosen degree, just like Cambridge, though.
Cambridge's system seems weirder to me - isn't it odd to accept the journey of someone who was potentially **** for brains 2 years before and somehow Brain of Britain 2 years later?