Blame the European regulators who decided that it was no longer necessary to have standard pack sizes.

Pack sizes were regulated in 1975 for volume measures (wine, beer, spirits, vinegar, oils, milk, water, and fruit juice) and in 1980 for weights (butter, cheese, salt, sugar, cereals [flour, pasta, rice, prepared cereals], dried fruits and vegetables, coffee, and a number of other things). In 2007, all of that was repealed - and member states were now forbidden from regulating pack sizes!

I think the rationale was that now the unit price (price per unit of measurement) was mandatory to display, consumers would still know which of two different packs on the same shelf was better value. But standard pack sizes don't just provide value-for-money comparisons, as this article shows.

Ironically it seems (from memory, I've not researched it deeply) that continental butter has not changed from 250g, whereas the British brands have moved first to 200g. I could understand if they switched to 225g as essentially a half-pound block, but 200g isn't any closer to an useful Imperial measure than 250g.