NASL's big problem was they over extended themselves. They got too big too quickly and didn't have a salary cap or anything to keep player costs from getting inflated beyond levels the teams could afford.
MLS has gone the opposite route, moving very slowly to open up spending rules, which at this point is a detriment to its growth in popularity, since it's clearly seen as well below the other soccer that's readily available (Premier League, UCL, La Liga, etc.)
Soccer as a whole is probably #4 as it stands, when you consider viewership of everything nationwide, including the Mexican league which is huge in that community. But MLS remains light-years behind the NHL and won't get close until they open up spending and really compete with Europe, to draw the Eurocentric viewers who look down at it.