Desdemona is a static character in this regard. Throughout the work, we see her remain the same, faithful to a flaw to Othello. She never does cheat on him, so his anger spent toward her is difficult for her to understand. I would also imagine that in the society in which she participated, women were very much expected to respect their husbands as we learn from some of Desdemona's own first words to us:
My noble father,
I do perceive here a divided duty.(195)
To you I am bound for life and education;
My life and education both do learn me
How to respect you; you are the lord of duty,
I am hitherto your daughter. But here's my husband,
And so much duty as my mother show'd(200)
To you, preferring you before her father,
So much I challenge that I may profess
Due to the Moor, my lord.
Even in Desdemona's dying words, she chooses to honor the Moor. He is killing her, and in the presence of Emilia she tries to do two things: clear her name of guilt for anything that people would accuse her of, and clear Othello of the possible accusation of her murder. Even in her death, she remains faithful.
Thus, my argument would certainly be the era causes this, but another idea might be the fact that she indeed loved him that much to practice true love, which is being completely self-less, even unto death.