Why do asteroids provide a better record of the Solar System's birth than Mars or the Moon?

They have sat largely unchanged, shielded from volcanic activity and tectonic shifting.

While bodies like Mars and the Moon offer insights, they are considered heavily processed worlds. Geological phenomena such as volcanic activity, erosion caused by wind or water, and large-scale tectonic shifting have effectively erased or obscured the pristine chemical and structural records dating back 4 billion years or more on these larger bodies. In contrast, asteroids, especially the smaller, dark C-type bodies, have remained largely undisturbed and chemically unchanged since the initial condensation phase of the Solar System. They have been shielded from these significant geological erasing mechanisms. Therefore, studying the array of asteroids provides a foundational view of the initial building materials available for planet formation, offering context necessary to understand why differentiated planets like Earth evolved differently than drier worlds like Mars.