People from the British Islands were only a small part of the colonists at first, with several other countries trying to grab land (in particular France, also Spain and Netherlands) and also with a lot of immigrants from other countries (in particular German principalities). But the British colonies were stronger and richer, supported by the powerful British Navy, so they ended up dominating. Then there has been some relatively aggressive policies of linguistic homogenisation, like in most European countries. Going from slaughtering French villages to forbidding French at school, depending on times. With the immigration of the 19th and 20th centuries of course the "English" part of the ancestry of USA became even smaller and smaller, while every new generation accepted to learn the dominant language.
That being said, USA has never had any official language, actually. And in many territories which used to be part of the Spanish Empire the Spanish language is now reappearing.