Why was 'u' invented?

Question

In the history of the letter 'v', Wikipedia mentions the origin of 'u' but unfortunately doesn't describe why it was created in the first place:

During the Late Middle Ages, two minuscule glyphs developed which were both used for sounds including /u/ and modern /v/. The pointed form "v" was written at the beginning of a word, while a rounded form "u" was used in the middle or end, regardless of sound. So whereas "valour" and "excuse" appeared as in modern printing, "have" and "upon" were printed as "haue" and "vpon".

The article on 'u' states the same thing:

During the late Middle Ages, two forms of 'v' developed, which were both used for its ancestor 'u' and modern 'v'.

It's a shame it doesn't go into depth about why the alternative glyph was developed.

What caused 'u' to start being used? Why couldn't ancient writers continue using 'v' for everything? Is there anything else interesting about the history of 'u'?

Answer 1

This addresses the last question, namely

Is there anything else interesting about the history of 'u'?

The Roman emperor Claudius introduced three additional letters to the Latin alphabet, among them the letter inverted digamma Ⅎ, distinguishing between the vowel V and the consonant Ⅎ. The Claudian letters were short-lived and not used after Claudius' reign. For more information on the Claudian letters, see also this question and its answers on latin.se: Examples of the use of Claudian letters (Ⅎ, Ↄ, Ⱶ)

Answer 2

I don't know of an explanation. V is not the only letter that has developed alternative forms based on its position in a word; this kind of development seems to just happen sometimes, without there necessarily being a reason why a single form "couldn't" be used instead.

In the Latin alphabet, another example is the development of ſ and s (although they did not end up being reanalyzed as distinct letters, and ſ just ended up replaced by s). Greek sigma σ has a special final form ς.

Five letters of the Hebrew alphabet have special final forms.

Answer 3

Find a pen or pencil and write the letter V over and over and over again. I guarantee you that some of them will be round bottom U's instead. With quill pens, it's harder - but not impossible - to make sharp corners, so medieval scribes had a higher proportion of u's to v's, and eventually said "screw it" and focused their efforts on more important instances of the letter, like word initials. By the time the printing press showed up, both forms were entrenched in the writing, so both forms were cast, much like the long and short S. From the there, the fact that /v/ and /u/ are fundamentally different sounds encouraged the modern convention.