First and foremost, EPL is the most popular league worldwide, which also happens to be Americans choice due to that fact that it’s English, and the most watched and broadcasted in the US (prior to early 2000s I think you could only get matches on satellite/ choice bars; don’t remember anything on cable).
As for the most popular teams, Man Utd and Liverpool, it’s mostly a byproduct of being the winningest clubs in the history of the EPL. Manchester City and Chelsea’s resurgence goes back 10 and 15 years, respectively, and the most popular choice with the nouveau fan (as far as EPL is concerned that is; the biggest club in the world and the standard for the winningest European giant is Real Madrid in Spain, but you can also make a case for Bayern Munich in Germany and to a lesser extent some Italians beasts that longer sustain that status such as AC Milan and Inter Milan).
Anyway, fans who followed EPL longer than the last 10-15 years know Manchester United and Liverpool are the winningest, most tradition-rich clubs in England, followed by Arsenal, and to a much lesser extent Aston Villa, Everton and Newcastle.