I think often there's a general distrust of decimalisation/metrication, and especially among older people. I remember my papa bemoaning the fact that when we switched from old pence to new pence, he got one less cup of tea from a two-bob bit. Pre-decimalisation he'd be able to by 6 cups at 2d per cup, and then it went to 2p per cup.
Another example, and this is from my own fairly recent experience: For years we had doorstep milk, delivered by a local dairy firm. It was mostly in glass bottles, and always pints. In the past they would occasionally deliver Tetrapaks, but it was still always a full pint. Then they started with the plastic "pint" bottles that contained 500ml and cost the same. Initially it was just the one bottle that was replaced by the smaller version, then two, and so on. The reduction in the quantity, coupled with the gradual reduction in the frequency of deliveries and the fact that it was far cheaper to buy from the supermarket, spelled the end for doorstep milk for us.
I still buy milk in 4-pint bottles, though.