Good stadiums tend to bring in higher attendance. Higher attendance tends to result in higher revenues. Higher revenues tend to result in better players. Better players tend to result in better football.
A more equal distribution of broadcasting revenue tends to make the league more balanced, making the seasons more exciting to watch. This is where La Liga fails.
Believe it or not, a lot of players care about the atmosphere. This where the Germans have a competitive advantage as well, having civilized fans that also produce a great atmosphere.
Why is the J-League where it is? Because it's in Asia. Players today want to play where there is strong football, especially in the Champions League.