Because it's either adjusted wrong or faulty. It's my understanding that if you're seeing voltage on the black/white wire, but it's not coming out on the black/red wire, then the fault has to be in the NSS.
Have you disassembled the NSS to clean it?
Did you check the black/red wire coming out of the NSS?
If your NSS is adjusted wrong or faulty, it's not going to pass current. It's supposed to pass current to the starter only in Park and Neutral.
How are you checking for power?
If I'm understanding the diagram correctly, you should be seeing ~12V on the black/white wire only with the ignition in the 'start' position.
You should see ~12V on the black wire with the ignition in the 'on' or 'start' positions.
The fuse block is powered by...
This often means the NSS is misadjusted. It could probably also be due to filthy contacts inside.
Have you disassembled your NSS to clean it, and then adjusted its position once it's reinstalled? Cleaning it isn't hard, just watch for the small springs that push on the copper contacts...