title_body: What reasons would there be for not having a human conlang with only vowels?
upvoted_answer: A vowel-only language is surely constructable (and I think, learnable, too), but I am afraid that it will be instable against evolutionary pressure.
  
  Vowel sequences like /aua/ or /aia/ tend to develop into glides /awa/ and /aja/ giving raise to the first consonants in the language, and at the hiat between two vowel a third consonant, the glottal stop, may materialise.
  
  Efficiency of the coding is also a concern, maybe tones can take over the role of consonants in creating more different syllables from the vowels.