Largely because it's super easy to play it informally in pretty much every poor village in the world.

1 - You don't need expensive gear. Just a patch of ground, one ball, and some stuff to drop on the ground to mark the goals.

2 - You don't need the full complement of players. While the official rules require exactly 11 players per side, you can cobble together an informal game with whatever smaller number of people you have available. (Compare with something like baseball, where the pitcher, catcher, infielders, and outfielders are significantly different jobs where people can't easily overlap between them so you need people at each position.)

This makes it so that poor countries actually have a shot on the world stage. Just because you grew up in a poor village doesn't mean you didn't get to spend a lot of time playing and practicing as a kid.