Good question, and I don't know the answer!
My guess, and it's just my opinion, is it was possibly a result of successful lobbying from the RHA or similar on the basis of economics for the Transport Industry.
Also, Commercial tyres are manufactured to different criteria, so the government...
I spent 35 years in the Tyre Industry in one capacity or another so I can probably offer a bit of helpful advice.
Firstly ... of all the advice given on this thread thus far, please do NOT follow the advice given above by nick.
Part worn tyres are a lottery.
You may be lucky, as nick obviously...