I imagine the reason these coins are uncommon is because Hong Kong was in the process of redesigning its coinage in 1980. A 20-cent piece had been introduced in 1975, and as this was smaller and lighter than the 10c coin, it would have reduced the demand for the latter (the same thing happened in the UK when our 20p was introduced in 1982). The 10c coin itself was replaced by a smaller, lighter coin in 1982. Maybe large stocks of unissued 1980 coins were sold to coin dealers.