Because she didn't cling on him like others women HAHAHAHAHA.
She was "different".
The same cliche
I'd say it was for something else. At first, around the time they got married, he liked her boldness and unique appearance and needed a wife. After that, they slowly got to know each other and he found her brave, wise and profoundly kindhearted. I think he also found her endearing because she sometimes is very collected, and some other times very flustered. And she was even accepting of his nephew/adopted son. It's all those things in my opinion that led him to be deeply in love with her
I'm re-reading and based on my observations, it's a mixture of things. However one of the biggest reason I've observed is Lucia reminds Hue of his brother. His brother was the first person to treat him normally and affectionately. Re'reading the scenes, you'll often see that whenever Hue is with Lucia, he recalls fond memories of his brother and the emotions/good feelings it brought him. His brother made Hue, "human". I imagine Lucia's actions towards him and Damian provoke similar feelings which gradually developes towards love.
Hue says many times that it's because she has this different reaction, and is kind of indifferent to him.
Even in the novel, he says multiples times
Why did Hue fall in love with Lucia, it's been so long and I honestly can't remember exactly why....