The answer of this question lies in the nomenclature of the late 1700’s when chemical knowledge was primitive. At the time, most chemists agreed on a class of acrid substances, for example vinegar, called “acids”. Another class of substances reacted with acids to form the “base” of compounds.
When systematizing these compounds, Lavoisier came up with the word oxygen by combining roots which translated meant “to generate acid” because he thought oxygen was used to form acid. Acid was also thought to be formed by hydrogen as well. Most of the early chemical formulas for compounds specified both hydrogen and oxygen as the acid forming component.
It was not discovered until 1832 hydrogen, unlike all of the other known elemental gases, shared a property with metal: it was electro-positive. So in the late 1800’s the nomenclature debate surfaced. Some of the old nomenclature was kept so the opposite nature of acids and bases was reflected in the nomenclature. This nomenclature was said to facilitate understanding of molecules combining and breaking apart.
As an example, notice how the nomenclature in the following reaction suggests two molecules with opposite properties are sticking together.
$$\ce{ NH3 + HCl -> NH4Cl }$$
The short answer would be early chemist developing nomenclature did not have modern understanding of hydrogen, so hydrogen often was placed in the same location of formulations as oxygen in similar compounds.
Sources:
Williamson. Remarks on Chemical Nomenclature and Notation. J. Chem. Soc. 1864, 17, 421–432.
Robert Kane, John William Draper, Elements of chemistry, page 152.
