In theory, sports leagues could increase the supply of tickets by scheduling more games. But in reality, adding games to the season is not a practical option, because demand varies widely from one market to another. Baseball tickets may be scarce in one market but not in another. And unlike many other products, the supply of pro sports cannot be tailored to meet the level of demand in a particular market. A movie distributor can choose to release foreign films only in big city markets where demand is strong, but a sports league has to schedule the same number of games for every team, regardless of how many fans the team is drawing.
There is also diminishing marginal utility to consider. How many games a week can a person watch and still have a life? At some point, there's the danger that a longer schedule won't hold fans' interest—especially in markets where the team is having a losing season.