Minimum wages have not increased in decades. Cost of living has increased a lot meanwhile, and the rich vs poor divide has increased. So lower class and middle class are suffering, while upper class have become richer from their labors. In earlier generations, the middle class could work for some years and afford to buy a house (on mortgage). But these days, middle class cannot afford a house, they live in rented apartments. Hosting parties is the least of their priorities, when they are struggling with the monthly bills.

Housing availability and leisure time (afforded by excess income) are probably the biggest components.

And in answer to "When that changed?" from parent, my guess would be mid-90s.

In that generations coming of party-hosting age after that were increasingly less likely to host.

I don't think it's that, my parents weren't two income and never had friends or did events or social things and barely left the house.

My mom would constantly complain she used to be a social butterfly but having kids "ruined" that for her. Which never made sense to me, it's not like she ever interacted with us much.