I've been thinking about this. My grandmother was a teacher and parent in the 1960s and 70s, and she was friendly with most of the neighbours. She had had years off when the kids were young (though she then went back and worked full time as a teacher) so maybe this helped give her more time to befriend the neighbours?
I've seen some articles say women not being temporary or permanent SAHMs as much is a factor, but I'm sometimes a bit sceptical as some seem to have an agenda of women working being bad. Also puts all the onus on community networking onto women rather than their husbands..
There's surely other reasons...moving is more common, for one. Collapse of community institutions which might have facilitated neighbour friendships more. And the lack of secure housing, renting more common etc In some cases, second homes meaning the residents aren't there a lot of the time (my DM' neighbours, for one)