There just wasn't a need for the U yet. The Romans added the Z to accomodate certain Grrek loanwords like zephyrus, "the west wind", but the sounds for V, W and U were all represented by the single letter V. The "rounded V", U, was found in late Latin inscriptions, but the differentiation of vocalic U from consonantal V wasn't found in written English until the late 17th century.

V was typically used in Latin writings, but it is transliterated into U by modern scribes when it functioned as a vowel. The English counter part would be W, as in "A E I O U and sometimes Y and W".