Lots of reasons: not claimed/collected, whereabouts of person unknown, not wanted, person never knew about them and so the list could go on. In the case of your two uncles it was only Thomas Mace's medal and additional clasp that were returned to the medal office. Both of them served in colonial units. It was a very common occurrence, especially amongst the colonials. I've found several such instances amongst my Boer War relatives.
thank you for your link but it seems you cannot claim medals any earlier than ww11,but it says we can get replicas from a dealer,im not sure about that,will have to keep scratching and thinking thanks again