The reason is because they are the people who will use the system the most. Any window manager developer who forgets that users are the ones using the interfaces should remove themselves from the project. Any window manager that is not being designed using usability testing and feedback from real world users will be doomed to failure. Another problem Linux currently has is the number of choices available. This includes the number of distributions, window managers and shells users can choose from.
Choice works well for developers but is bad for users.
Choice can only be good for users if changing an aspect of the system can be done easily and the new choice is consistent in the way it is used compared to the other method. If the choice means the user must re-learn several processes then even allowing the user the choice is a bad design decision. The way people in Florida will use the file browser will be the same as the way people in France will use it. While it could be argued that Microsoft locks users into one file manager the trade off is the constraint creates consistency between systems. The trade off between choice and constraints is a fine line.