From memory of a scholarly article several years ago, there was probably a lot more mass there to start with. Solar system building simulations tend to have several up to Luna size bodies form in the asteroid belt region which then either collide with each other starting the process all over again, or narrowly miss each other and get thrown into a Jupiter encounter which ejects them from the solar system entirely. I think it's 50-50 whether one of them wins out or we get the Ceres and rubble situation we see today.