Do you mean, why isn't it called "combined" even though the scores for both programs are added together like in solo dance?
I think it's because in solo dance the skaters can compete in only one of the two types of dances - rhythm (short) dance or free dance - or they can compete in both types. The combined score is for the skaters who compete in both types of dances. In "regular" ice dancing, the partners have to compete in both types of dances - they can't compete in just one of them.