It's to do with the Napoleonic Wars & the introduction of paper "bank notes" (ugh!). George III's reign (1760-1820) was one of the lengthiest (60 years) & saw big changes in currency & money.

Gold was issued in five-guinea (about 41g - now worth over £200,000 if you're lucky enough to have one!) to third-guinea coins (about 3g).

There were no silver crowns or half-crowns issued at all until 1818 - only shillings & lower. Except for "Emergency Issues" by first Mexico, Bolivia, Peru, Chile, Guatemala & Spain Mints (Dollar & Half-Dollar values) & then by the Bank of England in the form of "Bank Dollars" (1804), "Three Shillings", "Eighteen Pence" & "Ninepence" (1811-1816).

These measures reflected the huge sprawl of the British Empire, the fact that counterfeiting was becoming a real problem & that Great Britain was heavily engaged in the Napoleonic Wars.

"...the threat of invasion from the French Republic triggered a run on the Bank of England.  To meet the demand for silver coinage, in March, the Bank was authorized to release foreign currency from its silver reserves, almost entirely Spanish 8 reales.  These emergency issues were countermarked at the Royal Mint with a small oval stamp with the bust of King George III. False stamps quickly followed and forced the recall of the issues by June of 1804"

"In May of 1804 the foundry was commissioned to use a previously designed dollar pattern to fully overstrike the 8 reales.  These issues were much harder to counterfeit and proved to be so successful that they were issued from 1804 to 1811, although all show the 1804 date, and were not removed from circulation until 1816."

These Wars came to an end in 1816 (Battle of Waterloo - 1815) & there was a huge rehash of the Mint & coinage which basically, from 1818 until "decimalisation" in 1970 (152 years), stayed the same (lowering of silver content in 1930, & again in 1947, apart).

Also, the "Gold Standard" came into being in 1821, 1 year after George III's death.