Remember it was Dick Smith, when he was Chairman of CASA, who negotiated an agreement with the general aviation organisations (including AOPA) to mandate transponders in Class E. This was accepted by the GA organisations because it would give them the freedom to fly over Class D airports in Class E without a clearance.
The transponder mandate was introduced with Class E over D, but when Airservices reversed the Class E over D and replaced it with Class C, they left in the transponder mandate. It was an irreversible path towards more costs and more restrictions.
Tootle pip!!