>>49922295I don't know about the Gou x Allister doujin, but I do know the Gou anthology was sold at a convention, so maybe it's meant to be exclusive to those who attended? If it were available online, it'd deter people from going since they could buy it online.
It not being available digitally also makes it harder to pirate it, as evident by the fact picrel still hasn't been scanned yet. Alternatively, it could simply just be the artist's preference ("You have to be able to hold it physically to fully appreciate it").