According to our answer to the previous question, Mordechai refused to bow “to him” – in other words, to Haman alone, even without his idol. M’nos HaLevi writes that Mordechai was a very humble person, as befits a tzaddik. He would bow to everybody out of respect, but he would not even bow to Haman out of token respect. It should be noted that Mordechai’s refusal put his life in danger.
The Alshich writes that Mordechai did not bow despite the fact that Haman removed the idol he was wearing. Therefore, Mordechai no longer had the excuse that he could not bow for religious reasons, and yet he still refused to bow.