[jokoon]

For the same reason people talk english and not latin.

Even if latin is a better and more expressive language, choosing english as a language will make the project more viable.

A language is a method of communication, so it is a sound choice to pick a language that is understood by many, not the language that is better of more powerful.

Quick thought experiment: how probable is it that a project (open source or not) will die because it uses a language most programmers are not fluent with? How long does it takes anybody (meaning you include the not so good programmers) to learn python or lisp, and effectively be able to use it and solve problems with it?