Probably because of the British Empire. It was the last great empire, the largest ever and it didn't end officially until Hong Kong was returned to China in 1997. It is also the principle language of America (albeit in their own form), therefore the main language of computing and the internet.

However, note that there are more than double the number of speakers of Mandarin.

just To me , if America wouldnt speak English , English wouldnt be a spoken language this much.

just to me :)

I agree with Rob James I just want add: Speak English is more easy than Mandarin and English language are in 3rd position with 545 millons of speakers.

Because most of people speak in English~

I think it's more because English is the accepted language of business than anything else. $ runs a lot of things. Mandarin is the most widely spoken language of course, but for business it's English.