Because people buy a personal number plate for sentimental reasons; usually a a gift to a special person or to mark a special anniversary. There really is no need to stop these plates going onto an older vehicle any more. Anyone buying any vehicle will have access to the full details of that vehicle and the cherished plate will be replaced before sale anyway.
At present you can put an older plate on a younger car but not a newer plate on an older car. However now that all data is logged on the system so efficiently this is no longer necessary.
If you enter a vehicle's current registration plate into any auto website the original details of the vehicle's age, make,colour, date of first registration and even milleage in some cases is shown, regardless of the current registration plate it is carrying. This happens if you do this as an individual, a garage, insurance company and so on. When you ring the garage to book a service or MOT or order a part, they now ask for the registration number, as it appears on the car, and their screens will display the full and original details.
Obvioously folk want to take their personal registration plate with them to their next vehicle. They pay £80 for that privalage and the DVL/DOT then issue another plate for the original vehicle – in most cases it gets its own plate back. This is then logged into the system.
This law is not necessary these days. All the details of all vehicles are logged and any other modifications are (or should be) logged as well. There are after all lots of ways to make a vehicle appear younger than it is – a bit like the broom that you bought 20 years ago. It's only had 6 new heads and 7 new handles, but my, what a splendid broom it is!
Please sort this law out so that we can use our personal plates on any vehicle regardless of it's age. After all we not only pay for the special plate in the first place but we also pay to have it transferred and the details logged on the system so that they are there for all to see. That is of course absolutely correct. However, if not alowed to transfer it to an older car we then have to pay even more to keep it 'on file' (hanging up in the shed) and can never use it again unless we can afford to buy a car which is younger that the plate would suggest.
I feel sure that both money and time would also be saved in the departments who have to deal with this.