Mongolian U and the Shortage the Alphabet Inherited

The Mongolian alphabet has fewer vowel signs than the language has vowels, and the reason is historical. Its ancestor was an abjad, a script built for a Semitic language in which vowels were largely left unwritten and a single letter, waw, served for the rounded vowels and for the labial glide together. Sogdian and then Old Uyghur adapted that inventory as they went, and each adaptation improved the fit without completing it. Mongolian received a vowel system built for other languages and made it work rather than rebuilding it, which is the normal fate of a borrowed script.

Other scripts in the same position took the other road: Greek reassigned unused Phoenician consonant letters to vowels and produced a full alphabet, and Coptic added seven signs from Demotic for sounds Greek could not write. Mongolian added letters too, ten of them, but for consonants met in foreign words rather than for the vowels its own language distinguishes. That choice tells you something about what the script was for. Its users were not troubled by a written form that a Mongolian speaker could read without difficulty, and they were troubled by the impossibility of writing Sanskrit, Tibetan and Chinese names correctly in a religious literature that was full of them. The alphabet was extended where the need was felt, and left alone where it was not.