Afl & NRL are just the most established. It's coincidence they happen to be best in the world (although NRL for many years lost best players to rugby & Super League but that didn't hurt it).is it really though? Context is everything here.
In Australia its true - the Aleague suffers because people would rather follow the EPL and european leagues, likewise the NBL and the NBA. The NRL and the AFL are the peak of their craft and every piece of evidence shows that they dominate the winter period.
fu** knows about china and south america, and its not relevant.
What happens in the rest of the world is important because if what you were saying was true it'd be universal but it's not.