I think part of it is that household income has increased overall - women now work more often and at higher salaries than they used to, and other goods have generally become much cheaper. The leftover part, "housing + disposable income" is a larger proportion of the overall. As a result, people pump more money into the 'housing' bit - individually, it's 'have a bit less disposable income for a bit of a nicer place'. But at a demographic level, it'd put a lot more money into the market.

Totally a gut intuition, not supported by anything in particular. Things outside housing have become cheaper. And most people consume their full income in some way - I just think that housing seems to be taking up the slack from other things being cheaper.