My understanding is that MLS (that I do not watch and frankly, cannot stand) is profitable. So what does relative popularity really mean? It’s a business.
Minor league hockey is profitable too, doesn’t mean it’s popular, just that they’re smart enough to spend less than they earn. Relative popularity means benjamins. You can profitable English Premiere League hauling in 2.5 billion Euros from all revenue sources, or you can have a profitable NFL making as much money as the entire domestic movie industry anchored 5 billion dollars of TV contracts.