It's the number one sport in the world because you need nearly nothing to start playing it, practicing it, and becoming really good at it. And once you are really good at it, you have a chance to be discovered by a big team, and start earning a lot of money. 

Basicaly, it is an accessible way out of poverty for many people, all over the world. Even the poorest people can start playing it, with an empty sodacan, or even a rock. You don't need no expensive gear, no expensive to keep animals, no access to specific places...