Question

Why is `H_2S` called hydrogen sulfide instead of dihydrogen sulfide?

Answer 1

Historical usage, similar to why many element symbols are derived from their Latin or Greek names instead of the modern ones.

Explanation:

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).

17 . `H_2S`
dihydrogen(sulfide)

Note the difference from compositional names such as ‘hydrogen peroxide’ for `H_2O_2` and ‘hydrogen sulfide’ for `H_2S` (Chapter IR-5) in which (in English) there is a space between the electropositive and electronegative component(s) of the name.

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(`2^-`)’ can both be interpreted as `H_2S` as well as `HS^-` .

The situation with `H_2S` is completely analogous to that with `Na_2S` which may be named sodium sulfide, disodium sulfide, sodium sulfide(2 ) and disodium sulfide(2 ), except that misinterpretation of the first and third names as denoting NaS is improbable. In Ref. 2, the names ‘hydrogensulfide(1 )’ and ‘monohydrogensulfide’ for `HS^-` were proposed to avoid ambiguity. (However, in some languages there is no space in compositional names so that very delicate distinctions are required anyway.)

From: NOMENCLATURE OF INORGANIC CHEMISTRY
IUPAC Recommendations 2005
INORGANIC ACIDS AND DERIVATIVES IR-8.4
p.136