I was under the impression that binge drinking has been falling, particularly among young people?

I don't know enough about drugs to go into any detail, but (from the little I do know) decriminilisation or legalisation are not intended to increase consumption but are supposed to get rid of some of the really bad side effects - so it is not a case of not wanting to fix the problem but a different approach to how to fix the problem. Admittedly cannabis/weed seems to be considered differently because of how prevalent it is and how few side effects there are (comparatively).

You are correct about different attitudes to family and social liberalism, but I see that as an outgrowth of the need both to avoid discrimination and to limit authoritarianism. A left wing government by definition will have a larger degree of control over the economy and people's livelihoods; taking control of people's personal lives as well raises concerns of moving towards a police state (or nanny state as the conservative PR likes to put it). The alternative option of trying to give specific types of families incentives will always end up discriminating against those who do not conform (whether by choice or not).