It’s cliché to complain about how a movie or television show is ruining the source material by departing from the books. There’s nothing new about bitching that HBO is sabotaging A Song of Ice and Fire, the literary source for its program Game of Thrones, but what’s not being pointed out is why they are doing it.Cersei, in particular, has been sold short. As the writer notes: "In the book, Cercei Lannister is plagued by a mix of insecurity and self-delusion—Tyrion notes that his sister thinks she is “Tywin Lannister with teats.” Indeed, she looks up to her father partially because it enhances her own self-image as his equal. She also uses her sexuality as a weapon, betraying her brother (and lover) Jamie, who remains loyal. The show’s Cercei is portrayed as reacting to her oppressed status as a woman forced to marry men she doesn’t love."
The answer is feminism. Television needs to constantly reinforce the egalitarian narrative. The point of feminism is to absolve women from all responsibility for their actions. The show does this by creating simplistic explanations for the female characters’ actions and promoting Mary Sue style “strong women.”
Women in the books have complicated rationalizations for their actions, often deriving from deep seated insecurities and fears. Like real life women, they rationalize things to themselves based on deluded self-images, rather than reality. The show does its best to strip these away, the easier to blame everything on men.
It's not that the HBO Cersei is uninteresting; Lena Headey has presented an impressive character and been more than effective with the dialogue she's been given. But it's a little ironic that modifications made in order to make her character more palatable to feminists means that the HBO Cersei is neither as strong nor as ruthless as the book Cersei.