Oxbridge is a good place to be both a student and an academic because of the short terms. The academics get a chance to really concentrate on their research as well as teaching. This impacts directly on the undergraduates too, it means that the people teaching you are active researchers, in touch with the latest research in their field and constantly updating the teaching material.
Also, the fact that both universities take such care with the whole application process means that the interview period takes a lot of time and could not run alongside an undergraduate term.
And finally, conferences are an important source of income for colleges. Once the students have gone their rooms can be used for conference guests so shorter terms maximises the profit that can be made for this area. There are also a large number of summer schools etc which require accomodation in the colleges.