Because definite articles in Romanian are gender-specific. We use "o" for feminine nouns and "un" for masculine ones, therefore "o fată" was the only correct option.
When the noun is in singular, "o" is for feminine noun genders and "un" is for masculine and neutral noun genders.
O fată - două fete; Un băiat - doi băieți; Un lucru - două lucruri
(A girl - two girls; A boy - two boys; A thing - two things)