Because 120 years after the first recorded usage of Nitrous Oxide as an anaesthetic (and the intervening years when risk of hypoxia was reduced by mixing with air, then 21% Oxygen {i.e. the proportion of Oxygen found in air}, then various other proportions of Oxygen) BOC Medical trademarked their 50/50 concotion as Entonox in 1965.
gas - which comes from the word chaos - became attached to Nitrous Oxide by doctors who gave anaesthetics
and yes you could give gas-and air through something called a Minnitts apparatus 1935
and someone noticed that this would be a hypoxic mixture
and noticed that around erm 1962
and you will notice that three years later an oxygen enriched mixture got onto the market and presumably people said - oh goody ! gas and air is back !
If you read Minnitts 1935 monograph - you have to get one first - surprisingly enough the appendix at the back, he does do oxygen estimations on his patients using a van slyke apparatus and reports no hypoxia.
But only you and I know that one