Well, there's rather more to it than that.....
Oxford's station is situated to the west of the centre, whilst the bulk of the housing is to the east, through which the coach must pass on its way to London. En-route, it also passes one of the Park & Ride sites, which provides a fair chunk of the traffic as well.
I'd also suggest that Oxford has, over the years, had a rather more dynamic bus operation than Cambridge, which has provided a good level of base traffic.
Then there's the quality of the train operations themselves - a 3-car Turbo isn't the most inspiring unit available and doesn't compare well with the regularly updated coaches provided by the Oxford Tube & Espress (if that's what OBC are still calling it). Cambridge has 2 different lines so, if there's a problem with the Kings Cross trains, you can always go to Liverpool Street instead.