Probably management incompetence. The Costello legend has grown out of all proportion to the status of the cars in period. The Rover V8 was mounted on a standard B gearbox with an adaptor plate and a number of cars kept the B rear axle ratio.
Some later cars got an MGC axle (three different ratios available).
Costello showed BL what could be done but the factory cars were so much better engineered.
