Why were God’s commandments limited to ten? It seems that the additions of “Thou shall not rape," “Thou shall not condone slavery," and “Thou shall not abuse children" would all be appropriate additions to the list.
There are only ten commandments in the Old Testament because those commandments are a synthesis of the natural law. Other sins are classified by moral theologians under the existing Old Testament commandments. The additions you suggest would fit in various places under commandments 4–10, which deal with how to treat other people.