It is unusual to have Torah reading during the afternoon prayers and particularly uncommon to have it include a portion from the prophets as a Haftorah. It only occurs on fast days, not even on the Sabbath The haftorah read, including on Tisha Bav, is always the same, Dirshu from Isaiah fifty five. It is ultimate expression of repentance leading to the ultimate redemption. The initial verse hints at the period between Rosh Hashana and Yom Hakippurim, which should have led to it being read on Yom Hakippurim itself.
Strengthening this presumption is the view of the Mechaber, that after reading the Haftorah during Mincha that the blessings recited don’t mention that the day is Yom Kippurim indicating that the reading reflects that it is a fast day. Even the contrasting view of the RAMA which requires adding a blessing on the sanctity of the day doesn’t exclude the likelihood that the reading reflects the aspect of the day’s character that reflects it being a day of fasting.
The question becomes why we don’t read that portion and instead read the book of Yonah?