Question about digital outputs

Ok....I KNOW this is likely a dumb question since I am not all that well versed in digital stuff. 

So I just got a new integrated, including DAC. I wanted to run from the digital out on my SACD player (bypassing it's DAC) to the amp. But the player won't send the signal to the dig out when playing an SACD. It does with regular CD. According to the manual, this is  by design. So why is that? Again, I know there must be a logical reason but, as I said, I'm not knowledgeable about digital.

PS...the player is a Marantz.