My only answer is: That's just the way the software was written. To work around that limitation, I will add two combinations of less than 6 hydrographs, and then combine the two combinations into a single output hydrograph.
To simplify my explanation, let's say that I need to combine 8 hydrographs. To do this, I will combine two sets of 4 hydrographs, and them combine the two combination results into a single final hydrograph.