To tell the truth Angad's foot had more weight than the whole of Kailaash and Bhagwan Shiv, AT THE MOMENT.
Think what would have happened if Angad's foot had been uplifted. What would had happened?
As Angad was a great devotee of Sri Raam if Angad's foot had been uplifted, The name of Shri Vishnu and the trust of the name would have been considered false. As Raam ji's name can never be false and Moksha giving as true as death, no one in the court of Raavan was able to lift Angad's foot. It wasn't just the weight of the foot it had the weight of all existence. No one can except Sri Raam himself can uplift the weight of the Brahmaanda itself.
As Raavan was a param devotee of Bhagwan Vishnu in reality if he would have uplifted Angad's foot it would have meant that Shri Ram's name is false.
And, if Raavan would have not uplifted it it would have been a black mark on devotion. It would have been an insult to Prabhu Bhakti, the basis of God and his devotees himself, the basis of the universe. Thus, Sri Raam didn't made Raavan even try to uplift Angad's foot. Angad himself retreated his foot and asked him to touch Sri Raam's foot not his.