Because alcohol is taxed by the state and the repercussions of their use is generally paid for by the state.
Abuse of alcohol is a major problem, but you try to smuggle the equivalent misery through an airport. You can't.
These two knew that smuggling this amount of drugs would cause undue misery to a LOT of people but they didn't give a shit, they just wanted the money.
I agree that alcohol abuse in the UK is unacceptable but temper that thought with the utter misery that drugs, soft and hard, cause families and realise that these two were potentially adding to it.
Accidents abroad, fatal and minor, are linked to alcohol consumption and this will always happen because the state allows the sale of alcohol but it also provides services to deal with the aftermath through taxation. Drug dealers and their mules bear no responsibility for the harm they cause.
There are NO winners in the drug/alcohol problem but silly girls like these two should not be pitied, they knew what they were doing.