I suspect it is because too many people cannot hold their drink and create a disturbance which could put the plane in peril.
I am a drinker but think that is a fair excuse not to supply alcohol.
Alternatively maybe because it takes too long for the Trolly Dolly's to serve everyone and also due to the lack of room in the aisle.
I think you are referring to the bar service onboard - this is the duty free sales on board which will have little effect on drunkenness - anyone looking to get bladdered on duty free won't be waiting till they bring the trolley round on board, they'll bring it on from the huge duty free shop in the airport and get stuck into it from the off.
The idea that there isn't enough time is not credible, they never stop for the whole 4 hours - bar, food, duty free, bar, scratchcards - and the aisles appear wide enough each time too.