Because it's the best sport to play and to watch (yes, there are boring matches like in every other sport, but if you know enough of the sport you'll know which matches are not worthy to be watched)
On the other hand, it has the best structure... The American communist structure in sports is so fukking stupid, I can't understand why Americans like it... It's completely rigged and uncompetitive.
You suck? Good for you here you have the best draft... Rewarding the losers is vomitive.
Leicester City was literally pure crap for 100 years, they started to do things well in the last years and the last season with an outstanding scouting, much discipline and a lot of effort they managed to win the Premier League.
15 years ago, Barcelona was crap, but with a good youth system they managed to produce players like Messi, Iniesta, Xavi, Busquets, Puyol, etc, etc and now they are the kings.