For these content protection systems to work some parts of the device drivers and device specifications need to be secret. That makes writing open source drivers tricky in a technical and possibly legal sense.

I wonder why Microsoft is so fond of the idea then?

The AACS system hasn't been broken as such.

What has happened is remarkably similar to the original DVD hacks. One piece of hardware has been lax in its security allowing its firmware to be read and reflashed so it will give away the decryption key to anyone.