Why didn't the Reavers murder and mutilate everyone on Miranda?

In the movie Serenity, Mal and his crew travel to Miranda where they discover

Why didn't the Reavers murder and mutilate everyone on Miranda? Instead, everyone on Miranda

A plausible explanation is given in the Firefly Wiki article on the Reavers:

However, the article does not appear to cite a canon source for its explanation. Moreover,

Has a canonical source provided an explanation?