I think I'd pin the primary cause as the cost of living increasing rapidly and salaries not keeping up with inflation. Couple that with it becoming increasingly obvious that alcohol consumption has negative health effects and it's not surprising that people look for other forms of entertainment.

There's also just more to do in the modern world. There's things like raves as you pointed out, video games, the online world. There's also a million new hobbies which are now much more accessible thanks to the internet and online stores, and much more media content. Compare today's access to entertainment to the pre-television era...

And finally, to top it all off, we have technologies like instant messaging. Instant messaging makes it possible to group text your pals after work and decide to meet anywhere you like. Before that, coordinating with a large group of people was a lot more complex and took some planning ahead, so maybe it was just easier to default to always meeting to the pub after work.