The number zero did not originally have its own Roman numeral, but the word nulla (the Latin word meaning "none") was used by medieval scholars to represent 0. Dionysius Exiguus was known to use nulla alongside Roman numerals in 525. About 725, Bede or one of his colleagues used the letter N, the initial of nulla or of nihil (the Latin word for "nothing") for 0, in a table of epacts, all written in Roman numerals.
Additionally, on this article:
Although Romans used the word nulla (nothing) to convey the concept of zero, the Roman numerals lack a zero digit in their system.
This is interesting. If the Romans were aware in the mathematical concept of zero, or at least, some concept of "nothingness", then they would have been likely to have a digit for it. However, they didn't, as the two quotes say above.
Well, then what's the reason? Why did they only use unofficial symbols such as nulla or the letter N to represent the concept of zero, and not an actual zero digit?