because he thinks it won't be secure - because it's to be open-sourced. He believes in security throuch obscurity.
While publishing the source code might assuage Microsoft's competitors, it would be a boon to the crackers who want to find a way to evade the security system. The locks on your door and an alarm system aren't much help against a burglar armed with the system's blueprints!

Idiot. If knowing the system's blueprints was enough, your 'security' system was rudimentary at best in the first place.