Why do smaller bodies like asteroids maintain irregular, potato-like shapes?

Their gravity is insufficient to overcome the material strength of their composition.

The transformation from an irregular lump to a sphere is governed by the balance between gravitational pull and the structural rigidity of the material comprising the body. For smaller objects like asteroids, the mass is low, resulting in weak self-gravity. This gravitational force is not strong enough to overcome the internal material strength of the rock and metal composing them. Consequently, they retain irregular, non-spherical shapes. In contrast, planets large enough generate significant gravitational fields that enforce hydrostatic equilibrium, pulling all matter toward the center of mass until a roughly spherical shape is achieved.