Randi
Because most of the world has english as a language,so for the most part,people know english more then the smaller groups.There is,in short,a wider audience of people who speak english.
Piro
They probably feel like it will help them reach a wider audience. Most of the world these days at least understands enough English to get by, whereas if they spoke their native language, depending on what it is, they might be limiting their reach to just the people in their own country. That could be a difference of billions of potential viewers.
Jeff
To get more views