Why is it called Oxbridge when Oxford is obviously the greater uni?

Oxbridge is the portmanteau used to describe the two most famous unis in the UK. But whenever people talk about or think about a prestigious British uni they will only bring up Oxford and never Cambridge (this has been my experience in Europe, so I can imagine this phenomenon applies to an even greater extent the farther you travel away from the UK). 

I feel like Cambridge is often forgotten or completely overshadowed by Oxford, so why do people treat them as if they are the same when one is clearly the dominant member of the relationship?

Any measurable statistic of global prestige will be in favour of Oxford, for reference.

And will this translate into differences in IB outcomes, as another question?