Answer : Because they have less mass than planets and therefore less gravity
There are two main reason asteroids are formation of smaller rocks in space bound together and as they increase in mass and size, they also produce gravitational force but a lot less than planets, with such little gravity the same principle on planets where the heavier materials are closer to the core does not apply to asteroids, as such the surface is not compacted. Also the surface experience collisions with several other asteroids which produces the crater in them.
