בשנת מות המלך עזיהו ואראה את אדני ישב על כסא רם ונשא ... (ישעי' ו-א)
The Haftorah details how Yeshaya HaNavi beheld a vision of Hashem seated atop a lofty throne, the holy Merkava (chariot), in the year of the death of King Uziyahu of Malchus Yehuda. While the vision is awe-inspiring, what does it have to do with the death of King Uziyahu?
R’ Eliyahu Evron zt”l (Yosef Ometz) says that Chazal note how the Shoftim who led Klal Yisroel were often less than desirable. Some were boorish with no Torah knowledge. Others had limited Torah knowledge but were hardened warriors who glorified war and fighting over spirituality. Chazal explain that when Klal Yisroel is doing the mitzvos and learning Torah properly, they will merit leaders befitting their elevated status. But when Klal Yisroel falters, their leaders will reflect this level as well. However, Chazal note that a leader, even one not considered the loftiest, nevertheless has the ability to lead his generation to Torah and mitzvos, and if he manages to uplift himself and his people, Hashem’s joy will be immense, and the merit is that much greater.
King Uziyahu was a unique tzaddik who reigned in tumultuous times. Nevertheless, he managed to create one of the greatest restorations of holiness, second only to King Yoshiyahu, throughout Eretz Yisroel and Klal Yisroel prospered tremendously. Thus, the fact that Yeshaya HaNavi merited to experience such a vision of Hashem was primarily due to the righteousness of King Uziyahu. For this reason, the Navi added a seemingly innocuous fact at the beginning of the Haftorah.