A biographer has to get as emotionally close to his/her subject as possible: otherwise the writing won't come to life at all.
So it was primarily a desire to write about that period in one's life rather than that period in history or in British culture or whatever.
It's only a drawback in the States, where most people seem to have no real interest in other countries and the notion of a novel which might offer insight into life in the UK doesn't seem to appeal very widely.
I have to constantly rein in my nostalgia for the 1970s, in case it takes me over and I become one of those grumpy old men who just talks about how much better life was when he was a kid.