An excellent question! I wish I knew the answer. My best guess is that people who were not too fixed or settled in their pre-expat lives, who had lots of experience with travel and living in different countries before settling in the UK, are the ones who have the easiest time adapting to this country. Those who expect things to be mostly the same as back home, or who are constantly comparing the US and UK (usually to the UK's disadvantage), or who gave up a lot to move here, those seem to be the ones who struggle the most.
Largely, though, I think it's down to personality. I am one who had a pretty easy time of it, but I have wanted to live abroad since I was a small child. Before the UK, I lived in France, Japan, and Prague, plus all over the US. All that moving plus my longtime dream of living abroad I think helped me let the US go and embrace the UK. I do have occasional twinges of missing things, but mostly I don't think about what I've left behind. That attitude has served me well my whole adult life. I miss things from all the places I've lived, but I don't dwell on it. As a wise friend said on Facebook earlier today, "May the bridges I burn light the way!"