Why are Kuiper belt objects mostly icy rather than rocky?

Kuiper belt objects (KBOs) are primarily composed of ice rather than rock due to the temperature conditions present in the outer solar system during its formation.12

In the early solar nebula, a temperature gradient determined the composition of accreting bodies. Close to the nascent Sun, temperatures were too high for volatile substances—such as water, carbon dioxide, and ammonia—to condense into ices, resulting in the formation of rocky, dense bodies like the inner planets.123

However, at the distance of the outer solar system (beyond the "snow line"), temperatures were low enough for these abundant volatile substances to freeze. Planetesimals forming in this region incorporated these ices, which allowed them to accumulate significant mass. While some of these icy bodies coalesced into the cores of the giant planets, many remained as unconsolidated debris beyond the orbit of Neptune, forming the Kuiper belt.12