I think your answer lies in the primitive knowledge of disease and handicap at that time.There could have been some heredity factor which made them deaf, or possibly they became deaf from childhood infections in their ears, measles or scarlet fever.. However whatever the cause, if children are born deaf or become deaf early on they do not learn to speak properly, or at all, simply because they cannot hear. In those days they would just be classed as "mentally deficient" or "retarded" and often put in institutions - very sad. If you google Helen Keller you are bound to find something about her - she was one of the first people to be rescued from such a fate. Her family were well-off and she was lucky enough to have a carer who realised that she was not "mentally deficient" - she was in fact extraordinarily intelligent - and found a way to communicate with her. The story would interest you I think - but she was just very, very lucky for her time. This is the reason why babies are tested for hearing as soon as possible because the earlier deafness is caught the better the chances of a reasonably "normal" life. Hope you find this interesting.