I don't care one what or the other what language they have used.
They have chosen it because it is very popular language that is familiar with the largest number of possibly interested people, it's standardized, and ties into other technologies they are leveraging.
The point is that it's completely scriptable, so whatever type of window manager functionality you want you can spend your time concentrating on writing the functionality rather then writing a window manager.
But to each their own. If you want to write a new one from scratch then go for it.