Why did Marilyn hate the movie so much?

Where did you hear that she hated it?

Marilyn hated the movie because Miller constantly changed the script, giving Marilyn little to no time to memorize pages upon pages of dialogue. She would stay up all night trying to memorize her parts, and sometimes Miller would again change the script by the next morning.
Marilyn's marriage with Miller deteriorated for several reasons. Many sources say that the main reason was because Miller wanted so much to have children, and he would always get angry whenever Marilyn miscarried. Marilyn once said that her marriage didn't work because what Miller wanted was a trophy wife. He just didn't want to accept and deal with Marilyn's flaws and inner demons.
Marilyn's dislike for the film could also be partly because of the way the people handled the horses during the climax of the film. She was known to be a big animal lover.

Here's my theory: That Miller was writing a girl character that was more or less based on Marilyn herself... and every time Marilyn read her lines she knew that Miller still didn't understand her and didn't intend to start! I think he was more interested in his perceptions of her than the woman herself, as he proved when he wrote the dubious "After The Fall" (a semi-autobiographical play that I had to read in high school for some bizarre reason). And that's the problem with sharing your life with a writer, they might create a fictional version of you in their heads, and if they're as successful as Miller, they might share it with the world! Even after you're dead!

Plus, by all accounts, the shoot of the film was miserable. The Miller's marriage was falling apart, it was blazing hot and they were mostly out in the middle of Fuck-Nowhere Nevada, Montgomery Clift was drinking his career away and was hours late for his scenes, and Clark Gable was in declining health and would die after the filming completed. Legend has it that he never complained once, and went out of his way to be kind to his troubled co-stars, but he had to have been feeling pretty awful, adding yet another layer of misery to a miserable shoot.