Why did my topps 1991 complete baseball card set decrease in value over the last 20years... aren't they supposed to be worth more now?

Supply and Demand not Time

A collectible does not grow in value because of age. Supply and demand dictates the value. As time goes by, and more collectors join the market looking for an item that is in low availability the value will rise.

In 1991, the height of baseball card collecting, sets were produced in large numbers to meet the high demand. Today there are more sets or cards in the market than collectors. The value goes down.

Baseball cards produced before 1980 are in fewer number because in time cards were ruined, thrown out or lost. Today these cards are in high demand, and the suplly is lower than the number of collectors, raising the value.