The word was thereafter spelled with a lower case, c, to distinguish between Sovereign Citizens and federal citizens, the latter who became subjects of the federal gummint.
The 14th Amendment was written in order to grant privileges and immunities to former slaves who had no official status with the state or the federal government until those who decided to accept the protection of the federal government became its subjects . . . "subject to the jurisdiction thereof."
This should come as a surprise to many, realizing there are two classes of 'citisens' in America living side by side.
Yes, folks. The 14th Amendment only applies to subjects of the federal government.
So, where do you stand?
There is a more pedestrian explanation.
Legal documents of the Constitutional period regularly capitalized nouns, in the German style. This practice continued thru the mid-1800s, when it was dropped from the stylebook. Thereafter, the distinction was between proper nouns (capitalized) and common nouns (not capitalized) -- as is the practice today.
If you go back and look at the wills, deeds and land transfers of the Constitutional period, you'll find that nouns were generally capitalized. After the Civil War, they were not.
With the US Constitution and the law.