[flagged]
This led me down a minor rabbit-hole.
In Australia, at least, hard liquor is sold in 700ml or 1 litre bottles. Wines (not what I'd call hard alcohol) are typically sold in 750ml.
Why 750ml is the default for wine bottles seems to be uncertain.
It may be any or more or none of :
750ml is roughly 1/5 of a gallon
750ml provides 6 servings of a 'comfortable' amount (about 125ml)
glass bottles, when blown rather than cast, were limited by the amount of air a glass blower could push into molten glass - this was around that figure.
Either way, none of that, in and of itself, is a compelling argument for metric.
We still colloquially call 750ml liquor bottles a “fifth”, so this is a plausible explanation.
(Curiously, we never say “a fifth of wine” even though wine is sold in 750ml bottles too.)