Why is Meno's definition of virtue unsatisfactory for Socrates?

Socrates explains that it is not enough just to give examples of a thing in order to achieve its definition. For instance, if we were trying to define color, we could not just say that 'red', 'blue', and 'yellow' are colors. We would have to step outside of instances of color to look at color more objectively. Thus, a thing is separate and distinct from particular instances of that thing. This is the general gist of the Socratic method, and it eventually gives credence to Plato's theory of forms.