While the hearing was a significant event in the book, Dumbledore was focused on other pressing matters. He believed Harry's account of Voldemort's return and was working behind the scenes to prepare for the coming war.
Here's why Dumbledore was absent:
* He believed in Harry's story: Dumbledore had no doubt about Harry's truthfulness, so he saw no need to personally defend him.
* He had other priorities: He was preoccupied with rebuilding the Order of the Phoenix, preparing for the fight against Voldemort, and trying to protect Hogwarts.
* He wanted Harry to stand on his own: Dumbledore likely wanted Harry to face the hearing alone, as a test of his strength and character, and to demonstrate to the Ministry that Harry could handle himself.
Ultimately, the hearing was meant to show how the Ministry, under Cornelius Fudge, was choosing to ignore the truth and silence dissent, which was a key theme in the book.