Why Is the Letter "W" called Double-U Instead of Double-V?

The "w" in the English language is an outlier. It is the only letter with a name that's more than a syllable long (it has three), and the only one in which the name is a visual description instead of its sound. Yeah, it would make sense to call it wuh, but it's a bit too late for that. Furthermore, its description doesn't make sense in type. It looks like a double "v." The conundrum is the basis for a children's poem from 1885.

 “Excuse me if I trouble you,”
    Said V to jolly W,
    “But will you have the kindness to explain one thing to me?
    Why, looking as you do,
    Folks should call you double U,
    When they really ought to call you double V?”

    Said W to curious V:
    “The reason’s plain as plain can be
    (Although I must admit it’s understood by very few);
    As you say I’m double V;
    And therefore, don’t you see,
    The people say that I am double you.”

But if you want the real answer, you have to go back to the time when the Latin alphabet collided with the English language in the 7th century, and had to make accommodations for sounds that did not occur in Latin. For the "w" sound in Old English, they used two letters. And those were "u"s. For example, the word "wonder" was spelled "uundra" in Old English. The process of this sound becoming a "w" in English is a bit more complicated. For instance, there was another letter, "ƿ" (or wynn) that tried to take its place, but was ultimately discarded. You can read up on the evolution of the letter "w" at Grammarphobia.