I am watching some MLS games from time to time and think a good amount of fans/spectators is people with european and south-american roots. Now in the US most people have roots from all over the world, but I am more talking about people that live in the US in first or second generation. They are used to soccer being their main sport and probably all have a favourite team in their motherland and "live" soccer from a young age. Now residing in the US, they just look for some soccer stadium feeling and thus attend the games of their local team and have some fun.

The MLS is a very international league with lots of foreign players from all over the world, so these fans can also find some compatriot to support in the teams. And then there is some big names, like Zlatan, Schweinsteiger, who people like to see doing their job live and while still living in Europe were not able to.

EPL is of course the biggest league in all of football, so it's just normal that there is fans everywhere. La Liga being quite popular in the US is probably due to all the hispanic diaspora. It's easier to follow a league/sports if you understand the language it is mostly covered in, can read the news, join discussions in forums etc. And La Liga, with those two big giants Real and Barca are, always catches international interest of course anyway.