You cannot compare two competely different projects and tell me "Hey why not go with the old one". The SLS program was to use recycled Space Shuttle parts in order to save time and money, and asking them to add a second stage to the SLS will just break the compromise, since they have done the math where the core stage itself is already enough to get above 100 km no problem with the help of the boosters as sustainers, burning until the TWR of the core stage is high enough to become self-reliant.