I’m thinking what Hostage is thinking - that the boxes were left blank for citizens.
I have 1920 forms for my ancestors. For my ancestors who were immigrants, the citizenship boxes are filled in. For their children who were born here, the citizenship boxes are left blank.
You are probably right. Where my dad grew up (Bassett, VA) there likely weren’t large numbers of migrants at that time. It was a very poor community. He actually lived in a place called “Horsepasture” which even today only has 0.35% latino residents.