I'd say the nba has always been bigger than MLS it just was lacking star power and accessibility. Check, I'd say NASL was more populAr in its prime than MLS .Gotta disagree with you on NASL. Pele was in the league and pretty close to still being in his prime, so the Cosmos were huge. The league itself was not popular outside of NY. MLS is a much more successful league and won't be going away anytime soon.
You're absolutely right with MLS being more popular with soccer fans, but NASL did a decent job initially in bringing in non-soccer fans to watch their games based on that star appeal (Pele). They just couldn't sustain or build on the momentum. MLS is a far superior league, but it is still a niche.