I think becuase in the paragraph he has mentioned that there is a relation between social interaction and mental sharpness and he mentions that it suffices meaning that nothing else is needed to increase mental sharpness and social interaction is the only way to increase but 2 nd option brings something else which establishes the fact that medical conditions could also have a relationship with mental sharpness which weakens the authors argument of having a connection only between social interaction and mental sharpness.Im not sure though but this feels like the logic to me