Rating this film... why so low?

It is now slightly up to 6.4, which I agree is still too low. I think the complexities of the plot probably made many feel that it didn't make perfect sense to them. Can't think of any other reason, because it's certainly a very engaging and well made movie. Not perfect, but a whole lot better than many mediocre efforts scoring higher marks here on imdb.

No robots, superheroes or cgi fantasy.

well, i can quote from what gwen Watkins wrote in her book 'cracking the luftwaffe codes' about her time working at bletchley:
"I read Robert harris's novel Enigma, about Bletchley park, and this really did galvanise me into beginning my own book. i could not bear that readers should think BP was anything like that. Where were the glorious eccentrics, the brilliant wits, the men and women of outstanding intellect, even genius, who had helped to save britain and, some critics say, to shorten the war by two years?"