Why do so many speak English?

The main reason a large number of people speak English is the former widespread reach of the British Empire. India, Australia, China, Canada, and the United States were at one time largely or entirely under British control. Thus both native residents and British colonists used English in those areas.

Due to the success of these former colonies, English has been adapted as a second language in many countries, even if they have no former ties to the British Empire, for trade, diplomatic, and scientific reasons.