What prevented planetesimals in the asteroid belt from forming a single large body?

The immense gravitational influence of Jupiter.

The asteroid belt is not a collection of fragments from a destroyed planet, but rather a surviving collection of planetesimals whose growth was arrested prematurely. The primary factor that prevented these materials from coalescing into a single, massive world was the dominant gravitational presence of the massive gas giant, Jupiter. Jupiter’s immense gravitational forces exerted significant torque and scattering effects on the growing bodies within that orbital region. This disruption prevented them from achieving the critical mass necessary to sweep up surrounding material and transition into a fully differentiated planetary body, leaving the materials scattered and preserved in their nascent state.