Why Ravana could not break Shivadhanush?

If Ravana was a great devotee of Lord Shiva, why couldn't he move Shiva Dhanush even a little in Sita's Swayamvar?

The two aspects mentioned there in the answer, his arrogance and the sexual look down attitude towards women, made him not lift Shivadhanush.

It would actual surprise us that Lord Ram was not the first one to lift Shivadhanush. Sita was the first one to lift as the maids could not clean the bow properly, Sita facilitated the cleaning process by lifting it. King Janka was highly surprised. Thus, Paravathi was surprised too and asked answer to her question.

Now coming back to Parameshwar answer to his wife:

"Shivadhanush symbolises the character and commitment that must be showed by both husband and wife in a relationship. The Bow is represented by Male and the string is represented by Female. The bow is very strong and crooked and the string is strong as well cause it will hold both ends of the bow but portrays as slender. If the bow is tied tightly then the bow will break but if it is tied loosely then the bow in itself is useless. Thus, female holds the life in a relationship and male's , survived by the induced life, action governs where this relationship leads, strengthens and protect it. Hence, Ravana could not lift it by mere brawn power cause there is no commitment given.

Sita represents Life.

Lord Ram represents Commitment and Strength.

Ravana represents Arrogance and Ego.

Arrogance and Ego breaks the relationship by hammering into Trust which holds up the relationship.