Why didn't more people like this film?

Rating: 9/10 (17 February 2009)

The Duchess (2008), directed by Saul Dibb, is a period film with a difference--the events actually happened. Keira Knightley portrays Georgiana, a young 18th Century aristocrat who entered a loveless marriage with the Duke of Devonshire, played well by Ralph Fiennes.
The first surprise of the movie is that Knightley is able to look like the real Georgiana, Dutchess of Devonshire, whose likeness was captured by Reynolds and Gainsborough. The second surprise is that Knightley can act! (It's no surprise that she's strikingly beautiful in lavish period costumes, but I didn't know she had talent.)
I don't pretend to be an expert in English history of that period, but the outline of the plot is historically accurate. In fact, the only overt historical discrepancy I could find is that Charles Grey (Dominic Cooper), who is in love with Georgiana, is portrayed as relatively poor and low-born. In reality, his family was powerful and well-connected.
Hayley Atwell does well as Bess Foster, the Duchess's closest friend, and the incomparable Charlotte Rampling portrays Lady Spencer, Georgiana's mother. (Yes--it's the same Spencer family. Princess Diana in many ways reenacted the role of the Duchess 200 years later.)
I enjoyed the film, and gave it a vote of 9. The IMDb average for the The Duchess is a mediocre 7.0. This is an excellent film. Why didn't more people like it?