People call it the most accessible because as long as you have a ball you can play it, at least to some degree.

The first reason for the low percentage is that making a WC with every (or even most) countries in the world is not feasible, it would be too expensive and take too long, not too mention that in the early rounds the quality of play would be too low. The other reason is that football has still not achieved significant popularity in China and India. I think that sooner or later one of them will host the WC to try to boost its popularity.

A country will obviously have to invest a bit to be able to compete in a competition like this though, growing talent is no easy task, it takes years and obviously some money. But it's not like you cannot achieve moderate success without a lot of money.

The WC is expanding to 48 teams though, so the numbers are only going up.