I think it's because red hair is carried on a recessive allele (ie if there is a brown hair allele available from either parent, the hair will be brown; it will only be red if the red hair allele is provided by both parents), and so to get red hair you need two parents who both come from a very narrow population with a high degree of genetic similarity (the Celts). Therefore redheads also typically inherit the face shapes and other physical characteristics that are prevalent amongst the small genetic group to which they belong.
Conversely brown hair, for example, occurs in a very diverse group of genetic populations (Indo-Europeans, West Eurasians, Caucasians) and those genetic populations are highly hybridised, so brown hair is associated with a much wider range of face shapes.
Not an expert on any of that though; GCSE Biology was long ago.