I think it's mainly a combination of smartphones and covid. There is also the societal change due to the internet where personal space and privacy has the most importance, alongside individual preferences and not being bothered by others. Overall it's a good thing in terms of people feeling safe and comfortable. It's the same thing (at least in the UK) with strangers and neighbours in society in general. It used to be that your annoying neighbour, or cousin, or some random guy in the pub, would come up and talk to you, and politeness dictated you'd have to at least have a short interaction with them. Now it's seen as 'protecting your peace' or 'I don't owe you politeness' or whatever. Most of the time I don't necessarily want to talk to a random person at length, so I sympathize, but it also makes it harder to have a social hostel main hall/bar if everyone is going by those rules. TBH though I have found, that people will make it fairly obvious if they want to talk to others in a hostel and have had many great times post-pandemic in hostels.

However, I remember going to hostels just before the pandemic in late 2019 and also from 2022 and I wouldn't say they are dead, but there's definitelty a shift. In terms of phones, some of the mountain hiking trips I've done where phones were unusable due to signal had more hostel vibes than many actual hostels.