Why should you not overuse 'was/were'?
Using was/were too often means you might be slipping into the passive voice: "Why was the road crossed by the chicken?" The active voice paints a clearer picture: "Why did the chicken cross the road?"
Not every was/were is passive. For example, in the phrase "Christine was a good student," was describes a state of being. And using the passive voice may be a stylistic choice, especially if the subject is unknown or intentionally hidden, e.g. Minorities were marginalized.
But beware of using the passive voice too often, because it can reduce the impact of your writing. Only use was and were when they are really needed.