Why was the council of Trent one of the most important general councils in the history of the Catholic Church?
You question assumes that the Council of Trent was one of the most important general councils of the church. This might be assuming too much, because there were many councils that people can point to that might have been more important. For example, the earlier councils, such as Nicaea in 325 dealt with important theological concerns like the nature of Christ. These theological topics could have changed the church as we know it. In short, it is debatable that the council of Trent was one of the most important councils.
With this said, Trent was important because it came during the context of the Protestant Reformation. Many people began to revolt against the theological problems and the immoral practices of the Catholic church. Therefore, without the reforms of the council of Trent, the Catholic church could have gotten weaker and the protestant movement could have gotten bigger.