It must be housing and expectations on living standards plus commuting costs. Up until the 70s a fair proportion of people worked near to where they lived. A 90 minute commute would have been considered a bit bonkers but is commonplace where I live.
In my case we are getting by ok on a single salary of just 22k (plus about 2k tax credits) living in the southeast with a SAHD and my work being a bike ride away (so no travel costs).
BUT
only because we own our own home outright, thanks to DH buying in the 90s, overpaying his mortgage and then a redundancy payout covering the remainder.
We don't run a car because we live somewhere with good transport links.
We don't eat out or get takeaways or expect holidays (we've not had a holiday since 2011 but will be going away in the UK this year). We cook from a scratch (including that infamous chicken that does 12 meals - a roast dinner, a chicken pie, pie leftovers and a chicken soup for 2 adults and a toddler) because with one parent at home you've got time for that.
Because we're on a low income (especially during my maternity leave!) we cut right back and it amazes me the things shops and supermarkets stock that are unnecessary and expensive for what they are yet popular. A silly example: we only buy bars of soap. It's so cheap compared to shower gel.
There is, of course, nothing wrong with buying shower gel or ready meals or takeaways, but back in ye olden days when you could get by on a single wage these things weren't even in the shops in the first place. The expected standard of living was lower and nobody minded because there wasn't much alternative. I do think TV advertising likes to rub our noses in how lavish other people potentially live (those huuuuge kitchens in tv advertland, for example) and many people seem to think a foreign holiday is a minimum requirement for normal life.
That doesn't mean everyone can just easily drop an income earner. We can only do it because housing costs are taken from us. I would have to earn twice what I do to afford to live here if we rented, say.
It looks even more bleak for those in their twenties now. 