Why does Stella stay in the abusive marriage with Stanley?
Throughout the play Tennessee Williams makes it clear that Stella is crazy about Stanley even though, or because, he has such a violent, animalistic nature. She is in love with him. She expresses her feelings clearly in Scene Four when Blanche tries to persuade her to leave Stanley.
But you've given in. And that isn't right, you're not old! You've got to get out.
I'm not in anything I want to get out of.
Earlier, in Scene One, Stella makes such statements as:
I can hardly stand it when he is away for a night...
When he's away for a week I nearly go wild!
Obviously Stella is willing to put up with a certain amount of crudeness and physical abuse from a man who gives her such sensual pleasure and genuine love. There is no question of her leaving Stanley. If he has her in his power, she enjoys it. And it would appear that she has him in her power too, as exemplified in that famous scene where he shouts, "HEY, STELLA!" This is something Blanche could never understand.