Why does more than three-quarters of the universe's hydrogen remain unburnt after eons?

Fusion is confined to dense, hot stellar cores, leaving vast interstellar clouds untouched.

Despite the immense timescales over which stars have processed material, the vast majority of hydrogen remains because the process of conversion is highly localized and limited in scope. Nuclear fusion, which converts hydrogen to helium, only occurs under the extreme conditions of temperature and pressure found deep within the cores of stars. Vast quantities of hydrogen exist in the form of cold, diffuse gas in interstellar and intergalactic space, far removed from any stellar nursery, and thus remain untouched. Furthermore, the sheer magnitude of the primordial hydrogen supply created during Big Bang Nucleosynthesis was staggeringly large relative to the rate at which stars have been able to consume it over the observable history of the universe.