My thoughts exactly. Melted by the Mint due to lack of demand seems the most likely explanation, especially as no more large 10p coins were issued outside sets after 1981. When you think that 1952 sixpences, 1959S shillings, even 1946 3d bits in average grade, are relatively plentiful (and they are all mintages of only 1m or even less), you'd expect a coin of 3m specimens to be even more common and they just aren’t.