> Taxes had a lot to do...
Kinda? Taxes on the UK's wealthy (not just property taxes) skyrocketed from '14 to '46, mostly because the gov't needed to seize every farthing it could, to starve off national bankruptcy. And taxes were only part of the US issue. Wikipedia notes that just https://en.wikipedia.org/wiki/The_Breakers needed 50-ish servants to run, and 150 tons of coal a year to heat. Imagine the payroll and utilities to live at https://en.wikipedia.org/wiki/Biltmore_Estate
Worth noting - in Britain's "good old days", there mostly seemed to be no lack of heirs with the £££££ to staff and maintain those enormous estates, century after century after century. Yes, that was partly social. And primogeniture certainly helped.
In fact there was a lot of churn and drama. What we have left today in the UK is literally the result of survivor bias.
A lot of country houses are now state-owned, managed through nominal charities. Quite a few disappeared in the 20th century.
The British aristocracy is a complex thing, with came-over-with-the-Normans at one extreme, and relatively recent self-made opportunists at the other. It's a socio-archaeological phenomenon in its own right - influential, but under-researched, and opaque to outsiders.
Heck, a whole TV show about it :-)
What little direct insight into it I have in the northeast US, it gets very complicated especially as large extended families get involved. People have very different goals with largely differing amounts of money and taste for spending it. Best is to run away in my experience.
Oh I don't really disagree with any of that. In both the US and the UK, the interest in staffing and maintaining multi-million dollar high-maintenance summer places (typically in conjunction with multi-million dollar city places) definitely declined over time.
There are still some ultra-wealthy with multiple homes, but there are also probably increasingly options for people to just rent something for a month or two that's a lot less headache with some minimal staff (if that)--or at least have one of a couple of places be as low maintenance as possible.