Because there's a ton of business to be had with English speakers: buying, selling stuff, tourism. Also many of the English speaking countries are very cool to visit or even move to.

There's not that many Spanish, Chinese or even French speaking countries you'd want to move to, as someone from a developed country.

I think the head count is not as imporant as the number of affected countries and the attractivesness of doing business with the people in those countries. Another factor might be the difficulty of learning the language, as English is considered one of the easier lagnuages.