Why do Roman Numerals have English letters?
It's because the English language is in part derived from the Latin language and Latin was the language spoken by the Romans.
A) Arabic numerals are in numbers whereas Roman numerals are in letters. B) Even if Roman numerals are in letters the symbols are easier to understand, despite the fact that Australians and Americans and most probably you write numbers using the system of Arabic numerals. C) The system of Roman numerals was invented before the system of Arabic numerals, but people use the system of Arabic numerals to write more frequently.