It's possible the Mint was trying to anticipate demand. Before the mid-90s there wasn't the annual flood of commemoratives that we now find familiar. You had the new £2 coins in 86, 89, 94 95 & 96 and the occasional crown up to 1990, together with the revaluation of that item from %/- to £5. Once they became £5 they were simultaneously issued on an annual basis. In the case of the Sovereign, there weren't many collectors of modern RM output in the 1980s, so it is quite possible that none were produced because they didn't think they would sell. Proofs are much easier to sell to the general populace than a dull currency piece. As the sovereign wasn't a currency piece despite its nominal face value, any bullion issued would be based on perceived demand for storing gold. I think it is necessary to cast minds back 30 years when the public's collecting habits were vastly different to today.