Why Are The Mid To Late 70's Proof Sets So Inexpensive?

From 1968 to 1974 there are only five coins in the set.
From 1975 to 1981 there are six coins.

The 2011 set has 14 coins and the 2011 set you quoted has seven of those coins in silver.

It has mainly to do with the population and the qualityof those sets, Cameo and ultra cameo proof coins from the 70's which exhibit those qualities are not very common. I went through hundreds of sets at shows,dealers and brick and morar stores to find cameo coins I think it took about 10 sets busted open to create each year set that met anywhere close to cameo and ultras lets just say that the search continues for many years in the 70's for those coins, the other eason for the cost today is the mint has been steadily raising their prices for proof sets and all of the coins are ultras which in reality hurts not helps the collector It's somewhat understandable with the number of coins which are in each set now, but the price IMO is still way to high.