Power users are certainly important, but finding them is not a simple matter. Now, both the Earl and His Majesty have decided to entrust this task to Jidan, which naturally shows their immense trust in him. Indeed, it seems plausible that he might be capable of this mission. However, when it comes to actually carrying it out, it is not an easy task.
After careful analysis, one can understand why. Although power users are not a minority in the plains, the populace is significantly larger. Searching for them one by one is far from simple. And yet, if everyone were to know about this mission in its entirety, it would lead to another set of consequences.
The Fire Emperor will surely become aware that they are searching for power users among the populace to become military leaders. He would then act and try to find these power users to kill or imprison them, which would be an effective way to prevent them from going to war.