A mild-mannered man may make a good husband, but he isn't the stuff of fantasy. He wouldn't defy the heroine. He wouldn't lose his temper. He would be sweet and understanding, in agreement with her at all times. In short, the conflict would fizzle, not sizzle.
Powerful characters generate powerful drama.
No self-respecting woman would fall in love with a man who abuses her. Often, the historical hero is a tormented man who has not reconciled himself to his need for love. His defenses are so great he cannot admit to such a vulnerability , not even to himself. He interprets his feelings for the heroine as mere physical lust. Over the course of the book, she teaches him that softness and strength can coexist.