One thing I've always wondered is why Epsom Downs trains are routed by the long, long circle to Victoria via West Croydon, when they could cut the corner and run via Mitcham. I'm assuming this is because of conflicts at Sutton Junction. Nevertheless, would there be much demand in running them via Mitcham or even fast to London Bridge via Norwood Junction?
Priority for the faster services is given to trains down the Epsom route which clearly go further out to Dorking and Horsham and have a greater catchment area. Even trains terminating at Epsom offer the opportunity for passengers to connect to South Western Railway services further south.
Obviously there is the conflict issue. However, I note that the fast trains on the London Bridge route run to Epsom, despite the need to use the crossover at Sutton.
I suspect you are overestimating the demand on the Epsom Downs line which doesn't really go anywhere. I'm sure plenty of people change at Sutton or West Croydon for other services.
In all reality, other destinations are deemed more worthy of the direct trains and Epsom Downs is just a long reversing siding that fits the timetable for the stoppers.