If Ravana was a great devotee of Lord Shiva,?

If Ravana was a great devotee of Lord Shiva, why couldn't he move Shiva Dhanush even a little in Sita’s Swayamvar? As per valmiki ramayana, there was never such a grand swayamvar held for goddess sita’s marriage like it’s usually shown in tv serials. There was just a condition put by King janaka that whoever able to string the bow of lord shiva would be deemed to be worthy to marry their daughter. Here, ravana actually didn't heard about this challenge as he was living happily in his golden palace which was many many miles away from mithila (Even if he took part in event, he would have failed to shake the bow as everything can't be achieved through strength). Over the time, many kings came across various parts of country and accepted the challenge and tried to string the bow but none of them able to even lift the bow which goddess sita used to play when she was a little girl. One day, lord ram and lakshman entered the mithila with sage vishwamitra and it’s vishwamitra who forced lord ram to take participation in the contest and rest in history