[swagasaurus-rex]

Well the bigger the gravity well, the more matter available to mine. Asteroids only make up a tiny fraction of the mass in our solar system.

[JoeAltmaier]

Asteroids may make up the vast majority of available system mass. Kind of hard to mine the interior of planets, so that mass is irrelevant. The asteroids are bits of broken up planet internals, right there for the taking?