If I had to guess at reasons for that, the prime one would be that the ATIS says to give all this information "on first contact with Heathrow" rather than "on first contact with Heathrow DIRECTOR". I wonder if a simple change to the ATIS would solve the problem and also stop people giving the information (incorrectly) to the TMA controller.
In a similar way, I'd bet that if the departure ATIS told people to report the information to "Heathrow DELIVERY", more people would make the first call correctly. In other parts of the world, the ATIS letter is reported to "Ground" rather than "Clearance".
G W-H