The answer, purely and simply, is because she loved him. There was no forced marriage or unfair setup - from what Ibsen tells us, it's clear that Nora actually loved and cared for Torvald deeply.
In fact, the play centres around an act of love: when, early in their marriage, Torvald fell ill, and his doctor insisted that he took a very costly vacation to Italy to get better. Torvald refused to borrow the money for the vacation, and so Nora borrowed it secretly, pretending it was her father's; and has managed, secretly, to pay the installments.
Nora does not hate Torvald - but, discovering her independence in managing her loan, gradually falls out of love not, in fact, with Torvald himself (though this is a symptom), but with the whole system of patriarchally-controlled marriage, with the whole idea of marriage as it existed in Ibsen's day. This is part of the reason why her final exit - and their parting - is so painful. It doesn't come out of malice.