I don't know whether I'd be classed as a young person for the purposes of this discussion, but I'm 28 and from my perspective, and looking back to my teens/early twenties, I'd say it's for a few reasons.
We're more aware, possibly, of the negative effects of alcohol these days. Campaigns, documentaries, social media, etc. I think that has stopped alcohol from feeling like just a natural normal part of life in the same way it maybe did in years past.
Social trends do tend to come in waves. I think for a lot of us, our parents were more hard drinking in their youth and at uni, and maybe tended to drink habitually at social events and on an evening. The response to that, as a trend, is that the next generation/s veer the other way and question the role of alcohol in their life. Maybe the next few generations who are parented by us will swing back more the other way.
Money. Nights out are fucking expensive. It's always been thus, I suppose, but it seems harder and harder to have a genuinely cheap and cheerful night out. EVERYTHING is expensive, even when you're trying to stick to cheaper drinks. The expense kind of takes the shine off what used to feel like fun and freedom.