Because it technically won't work. The Boot CD has to be of the sort recognised in pre-boot mode by the PS2. I'm shaky on the full detail because it's not published but the PS2 first reads the CD/DVD off-phase to establish from its colour (and I believe by other light related means) whether this is a PS CD, a PS2 CD or a PS2 DVD. Such CDs are created through a SONY OEM kit and pressed, not burned.

A copy is not in the necessary pre-boot condition. Now do you believe me?