As with most things in life, there isn't a single and simple answer.
Factors Oxford has (some of which have been touched on)
relatively indifferent train service to / from London - often very overcrowded inner suburban spec DMU trains
Oxford is one of the very few places where bus deregulation has led to long term, high quality, competition between operators, rather than incumbent v cheap and tatty challenger, leading to short term but unsustainable cost / price cutting.
two way traffic flow - tourists from London going to visit Oxford not just commuters from Oxford going to London (to an extent, the remaining coach routes serving Luton and Stanstead airports and Windsor, share this feature.)
high demand for student travel - so demand is not concentrated just round the peak London commuter flows.
The latter two make quite a difference to the economics of it all - while some of the costs of operating coaches are directly proportionate to time and mileage on the road, there are costs which are 'peak vehicle' related - primarily the cost of buying / leasing / garaging the coach, which are the same whether the coach is earning money for 4 hours a day 5 days a week or 12+ hours a day 6 or 7 days a week.
I have heard it argued that the latter issue, and failing to address it, was one of the downfalls of Southend Transport's coach network - there was a limit to the amount of daytime fill-in work in central London for the coaches, hence expensive coaches, and drivers being paid (not unreasonably) for a full day, not doing anything for much of the day.
I think most of the former Southend network of coaches is extinct - Stephensons took some of it on, but this ended a while ago.
Parts of Kent still sustain commuter coaches - a lot of the residential areas of the Medway towns and Gravesend are as convenient for the A2 as they are the town centre railway stations, and many of these routes offer direct services to the Canary Wharf area, which may be more attractive than getting on a slowish train and changing at Lewisham / Greenwich.