Aluminium cans weight a lot less than glass bottles. Coke bottles tend to have fairly thick glass as well because it is pressurised. The liquid inside weighs the same for the same quantify of drink. Less jet fuel need when loading cans on planes. OK, so the glassware is make of glass but then glass cups remain on the tray table and touch the passenger's dainty lips so quality must be preserved. Coke bottles are poured then whisked away. Glassware tend to be thinner (as there is no pressure to contain) and therefore lighter.

That is why most airlines now use plastic cup in Economy along with the fact that the table trays in Economy are much smaller so cups can fall off more often causing health and safety issues should they break.

Gawd! Does this sound geeky or what.