John Jarndyce, Esther Summerson, Rick and Ada just aren't interesting enough. The tragedy of Richard Carstone- which ought to be the emotional heart of the book- simply registers as one of its many incidents.
Dickens' decision to write as Esther Summerson is an interesting experiment. Esther's voice is largely convincing- if a little irritating at times- but to make it convincing Dickens has to dumb down.
As if to compensate for the constraints of vocalising Esther, Dickens cuts loose in the sections of omniscient narration. These contain some of the most sustained, the most gorgeous, the most brilliant writing he ever did.