I suspect the reason was because ,even though they were produced in large numbers, they were hoarded - GB plc was in real trouble at the time france looked in real danger of either passing their revolution to the UK or later on with invasion from napoleon. The american colonies were lost so a pottentally a large source of tax income was no more. And finally the king was behaving very strangely A lot of the coins in circulation were old and very worn (how often have you seen William III silver with no clear dates). The old addage of bad money drives out good , rang true here with everyone in the country trying to get hold of these coins of good weight and high silver content and salting them away under beds etc , just in case the worst happened.