Maybe simulation is useful in determining the content of others' mental states, whereas the theory theory may provide a framework for knowing when to attribute mental states and how mental states differ. Maybe they explain different things. TT has its strength in explaining conceptual change in development, whereas ST concerns itself with on-line use. ST argues that your own experience is extremely important in understanding the mental states of others. TT, on the other hand, argues that you can't understand your subjective experience without a conceptual framework.
Young children are typically poor simulators because they tend to respond egocentrically.