Why cancel a train part-way through the journey?

As a regular Dorking-Waterloo commuter, I find it really frustrating that SWT relies on Southern services to pick up passengers when it wants to clear its timetable and cancels the train at Leatherhead, 8 minutes from the final stop.

This only happens when the trains are already running very late, and therefore adds to the delay of the passengers trying to get to Dorking, as they wait for a Southern train to arrive (can be up to 30 mins).

I understand that congestion needs to be cleared, but the message it sends is that your service timetable is more important than your passengers.