Great questions, @Teriyakieverything
To start with, some students only realise too late that they are strongly suited for a campus or urban environment only, and they have chosen the other one. The size of the university/unit of admission/course intake can be another important factor - some realise too late that they need the buzz and multiple options of a large place; others that knowing and being known by their cohort and their lecturers is very important, that they need a sense of friendliness. (I am not saying large places are unfriendly! But usually you must put some effort into finding your tribe and some find this very difficult).
Pedagogical considerations are another issue, particularly for highly able students. Some genuinely have strong ideas about how their subject should be taught and do best in a programme that conforms to them. However IMO students who are unhappy for other reasons will sometimes pick pedagogical arguments with their curriculum almost as an attempt to justify their unhappiness.
I was educated abroad but know a number of Oxbridge students, some very well. One’s self confidence is a key factor in one’s happiness there, both with the tutorial system, which is very intense, and regarding one’s relationships with other students. Many arrive either under confident or over confident. Both can be very difficult.
Also, it is probably true that students who have come from certain schools and feel that Oxbridge is their birthright may, deliberately or not, intimidate some of the under confident ones from different backgrounds. It is easy to imagine that this just adds to the misery and indeed we sometimes see such postings on this Board. There is a bit of this at a few other universities, but I think (and the Board reflects) nothing like as much.
Ironically, some of these ‘Oxbridge is my birthright’ students may have overachieved in their exams owing to excellent educations and given excellent interviews owing to outstanding preparation, then turn out to be relatively weak students. There are many ways things can go wrong. This is why for the most part I don’t believe in overzealous preparation. You want to end up someplace where you will genuinely thrive