It's dying because their fan base is made up of mostly old retired people:

According to Nielsen, though, half of baseball's fans are 55 or older. A full 76% are older than 34 and only 18% are a race or ethnicity other than white. The average age of viewers during last year's World Series was 54.4.

Meanwhile, 40% of MLS' viewer base is 34 or younger. A full 14% are younger than 18 and 34% are Hispanic. Translate that to a World Cup viewership and FIFA says more than 50% of U.S. World Cup viewers in 2010 were age 34 or younger. Baseball may have a huge head start -- adding its last expansion teams in 1998, when MLS was just three years old -- but soccer's future in the U.S. is looking far brighter.

http://www.thestreet.mobi/story/12752071/1/5-major-league-soccer-teams-outdrawing-baseball-in-2014.html