Intuitively because if you're just writing zero bytes everywhere, and there's a lab that can detect the difference between a zero that used to be a one and a zero that was always a zero, they can get the entire drive.

When I get a drive, I LUKS encrypt it and fill it with zeros before throwing a filesystem on it, which results in the drive looking entirely like random nothing. The privacy test that it definitely fails is that disks don't ever look like that unless they're encrypted, but I'm not worried about anybody finding old drives.