It’s a long story. In short: In Latin script u and v were the same letter “u” but had two pronunciations depending on whether it was being used as a vowel or consonant. But when adapting the alphabet to Germanic languages (including Old English) the same two sounds were from two different letters, so they put two "u"s together to make double u: vv.

The full story: https://www.youtube.com/watch?v=sg2j7mZ9-2Y