Why is the Kuiper belt shaped like a flattened disk?

The Kuiper belt is shaped like a flattened disk because it is composed of icy small bodies that revolve around the Sun in orbits that lie close to the plane of the solar system.1

These objects are considered to be the leftover, unconsolidated debris from the formation of the outer planets. Because they originated from the solar nebula—the gaseous, disk-shaped cloud from which the solar system condensed—their distribution remains flattened toward the solar system's primary orbital plane. While the belt is generally described as a flat ring or disk, it contains various orbital subpopulations, including those with different inclinations, which are the subject of ongoing research into the evolution and gravitational history of the outer solar system.123