The Structure and Interpretation of Xmonad

http://xmonad.wordpress.com/2009/09/09/the-design-and-implementation-of-xmonad/

>>8
Because it's quite unplesant to make changes to the language or its libraries when you know your huge userbase is going to nag you incessantly about it, or when you start to weigh the changes you want to make against your fear of alienating part of the userbase. At least that's the argument they used to explain it, IIRC.