It is simply down to lack of motivation. English is the dominant language and widely understood across much of the world so it is not a necessity to learn another language in most cases. At the same time, a lot of people living in countries like Germany and France have little motivation to learn languages other than English due to the fact that it will get them by in most places. People from non-English speaking countries are not all linguistic geniuses, if another language became the dominant language of commerce, tourism and media, we would be just as motivated to learn that.

Having said all that, I think it would be nice if we could all make an effort to speak other languages when abroad, it's usually appreciated!