Let us examine a very suggestive midrash. It means his heart was haughty and did not say shira. Has not the sun stood still in the center of the heaven, and God's miracles were seen from one of the world to the other. Chizkiyahu, the personality whose heart is not able to say shira, does not see why the well-known historical facts of God's miracles need to be repeated every time God performs another one. In other words, shira does not carry any new information.
Prose states the timeless truth.
Shira expresses the truth of this particular second, the immediate reaction to the unique moment in history. To the prosaic eye, there is nothing essentially new in the downfall of Sancheriv that was not already demonstrated in the drowning of Par'o. To the shira personality, the defeat of Sancheriv requires an immediate reaction, for the truth of this moment is unique. Shira is spontaneous, prose is eternal. The Tokhecha in Ki Tavo expressed the timeless truth of the covenant between God and the Jewish people.