The bigger question to me is why it wasn't more popular earlier. Americans love sports, and soccer doesn't take any more sophistication to appreciate that baseball does. The World Cup gets massive coverage here, and each one brings exponentially more people into the fold, along with the expansion of MLS into new parts of the country (like the South). The MLS itself is a very well-run league, with a good mix of marquee players to attract people and homegrown and domestic players to keep the league sustainable.

There's probably also a hipster factor, lots of people who actually love sports but don't identify with the college football / NFL / NBA crowd.