Because microsoft does not support it. You might consider using a resource domain like ocs.211.calt- Belgian Exchange Community : http://www.pro-exchange.be -
Resource Domain can still apply in Windows 200x It is just a naming actually!
Other option would be migrating away from your domain to another domain. Or you could do a domain rename but that is a completely other ballgame which I am not in favor of- Belgian Exchange Community : http://www.pro-exchange.be -