Why is #H_2S# called hydrogen sulfide instead of dihydrogen sulfide?
Historical usage, similar to why many element symbols are derived from their Latin or Greek names instead of the modern ones.
Both are actually correct. For consistency, IUPAC has defined the "parent molecule" as "sulfane" with acceptable "official" names of both "hydrogen sulfide" and "dihydrogen sulfide" (actually preferred, both are "Compositional names" per IUPAC).
Note the difference from compositional names such as ‘hydrogen peroxide’ for
Compositional names of the above type, containing the word ‘hydrogen’, were classified as ‘hydrogen nomenclature’ in the discussion of oxoacids in Section I-9.5 of Ref. 2, and such names were extensively exemplified. However, in order to avoid ambiguity, their general use is not encouraged here. Consider, for example, that the compositional names ‘hydrogen sulfide’ and ‘hydrogen sulfide(
The situation with
From: NOMENCLATURE OF INORGANIC CHEMISTRY
IUPAC Recommendations 2005
INORGANIC ACIDS AND DERIVATIVES IR-8.4