The first possible answer is that there is no reason she should not have written a detailed study of provincial life. Provincial life was the subject of many of the great novels of the Victorian period.
Another possibility is that she was writing about what she knew. She herself was from Yorkshire, and brought up in an upper middle class environment of the type described in Middlemarch.
Finally, Middlemarch was actually an historical novel, set many years before the time at which Eliot actually wrote it, and attempting to analyse in part, the effect of the first reform bill. Many of the issues concerning reform involved the transformation of the power structure of the countryside and elimination of rotten boroughs.