It's a mix of all sorts of things, CB in the UK was really popular when there was an element of 'sticking it to the man' but legalisation gave it a huge boost, tens of thousands of people bought radios and within a year an awful lot of them were in cupboards, lofts, sheds or on the tip.
It declined from there with the rise of mobile phones and the 'net, in the UK telephones were *expensive* back in those days because there was one state provider and no competition, you weren't allowed to use your own equipment on their network, it was a criminal offence and you had to be rich to have a second line just for social use so CB was a cheap way to communicate between friends in your town, nowadays, messaging services like Whatsapp etc. are the favoured way for groups to chat and offer privacy and all the other benefits of a 'net messaging service.
The massive boost given to mobile phones by SMS was a big reason why people didn't take up CB, why bother waiting for your mate to be on air when you could text him any time, plus the rather naff image of American truckers from the 70s (think Convoy) and CB, radio in general, has become a bit of a joke to the general public nowadays.
I still listen in to CB occasionally (usually only when repairing one) but if there's anyone to listen to they're usually drunk, stoned, swearing at someone, selling drugs or some combination of, it's not really worth the effort.
These days I prefer to spend my radio time tinkering with and building things for ham bands or repairing things for people, gives me far more enjoyment.