Comment by smashers1114

A more physical way to think through this may be: imagine many many bits of rock all with their own semi random velocity. The overall system has an angular momentum that must be preserved. So as these bits of rock accrete you end up with orbits of large bodies that all must add up to the initial angular momentum of the system. Since every planet is a conglomeration of many bits it is extremely likely that all orbits are in the same rotational direction, though not guaranteed.