It's simply field experience that interactions are more typically subtler modifiers of strong main effects – here's a relevant quote from the second source:
>Unfortunately, very few times in psychology do we add a factor and expect a complete reversal of the effect. (...) [It’s] more usual that we expect the new condition to alter the size, rather than direction, of the existing effect.
Or from the first:
>[Whatever] power [simple effect study] had, at least twice as many subjects are needed in [interaction study], per cell, to maintain it. We know this because we are testing the reduction of that same effect.
It's simply field experience that interactions are more typically subtler modifiers of strong main effects – here's a relevant quote from the second source:
>Unfortunately, very few times in psychology do we add a factor and expect a complete reversal of the effect. (...) [It’s] more usual that we expect the new condition to alter the size, rather than direction, of the existing effect.
Or from the first:
>[Whatever] power [simple effect study] had, at least twice as many subjects are needed in [interaction study], per cell, to maintain it. We know this because we are testing the reduction of that same effect.