Because the Latin word for the Roman numerals XVIIII is novemdecim which literally means nineteen (19) and the Latin word for the numerals IXX is undeviginti which literally means one from twenty (20-1). Check it out in a Latin dictionary. The numerals XIX (there's no Latin word for this arrangement of numerals) are supposed to emulate the Hindu Arabic numerals 19 in subtractive notation. But the Roman numerals representing 19 in subtractive notation are IXX (20-1).