Because then vending machines would accept Euro coins as well as pound coins and you would be able to obtain goods for 60%(?) of the asking price??
But, the old UK 5 pence and German 1 mark coins had quite similar dimensions, and were made from the same material, so vending machines at that time were fooled by the 5p, which was worth much less than the mark. I understand that truckloads of coins were sent back to Britain as a result.
The vending machine use setup of electromagnet to recognize the coin, not their size. [1]
[1] http://someinterestingfacts.net/how-do-vending-machines-work...
*86%