One reason why sports tickets have become so much more expensive

I also believe that the major sports leagues NBA, NHL, etc. have government granted monopolies. Then of course they get our money through subsidies to build stadiums too, so we're essentially adding to the power of their monopoly by subsidizing them.

Also:

Baseball, of course, has long had more monopoly power than the other leagues thanks to a famous (or infamous, depending on your perspective) 1922 Supreme Court decision in which the court ruled that baseball was a sport, not an interstate business, and therefore not subject to federal anti-trust laws.

NY Times:

UNTIL the 1970s, Major League Baseball was a populist sport. Bleacher seats cost as little as a dollar, meaning middle- or even working-class fans could afford to take their families to a game a few times each season.

But in the years since, tickets to baseball games — along with other professional sports events — have skyrocketed in cost. Over the last two decades, the average ticket price for a Chicago Cubs game has increased 265 percent, more than four times the inflation rate. Add in parking, concessions and souvenirs, and a family trip to one of this week’s opening day games could easily cost a few hundred dollars.

There are many reasons for the price explosion, but a critical factor has been the ability of businesses to write off tickets as entertainment expenses — essentially a huge, and wholly unnecessary, government subsidy.

These deductions have led to higher ticket prices in two ways. On the demand side, they have fueled competition for scarce seats, with business taxpayers bidding in part with dollars they save through the deductions.

On the supply side, the large number of businesses bidding for expensive seats has driven the expansion of luxury skyboxes and a reduction in overall seats in new ballparks.

While baseball parks built in the 1960s and before held as many as 56,000 seats, the modern trend is toward smaller-capacity parks, with a higher percentage of total space dedicated to skyboxes. The new Yankee Stadium, the only major-league park built since 2000 with more than 44,000 seats, has 3,000 fewer seats than its 1923 predecessor but almost three times as many skybox suites.