The title of the novel is the name of the town, which is a hint that this isn't a novel about one or two individuals: it's a novel about the entire society of this small town and the surrounding area. Eliot's interested in showing how the relationships between different people have large and small effects on everything around them. The society of Middlemarch is like an intricate web of relationships, and it's hard to put any one person or group at the center of that web.
Society is to blame for the dissatisfaction that pervades Middlemarch, because characters are unable to reconcile their ambitions with the contingencies of social expectations.
The huge social web that Eliot develops in Middlemarch consists almost entirely of members of the upper middle class. Eliot is more interested in showing the intricacies of social relationships within the same class than she is in demonstrating the interrelationships across a more vertical cross-section of society.