You could look at it in the same way as when BE came in with an E195 on NQY-LGW when the railway line was washed away at Dawlish
NQY/LON/NQY is an established, and successful, route of some 50 years, or whatever, operations. that the, then, operator put some larger equipment on to meet a surge in bookings because of a specific problem.
On the other hand CWL/LCY/CWL has little, if any, history and when the train is quicker it shall never be a commercial success and is just in temporary place because of a specific problem.