It has to do with evolution. They first discovered the phenothiazine neuroleptics, chlorpromazine being the big break-through. Then they made all imaginable variations of this tricyclic structural backbone. One such variation was imipramine, which showed antidepressant instead of neuroleptic activity and became the mother of all tricyclic antidepressants. Later, high throughput screening was developed which allowed the pharmacological evaluation of a much larger amount of diverse molecules compared to the earlier days when each molecule had to be tested in animal experiements. From then on pharmaceutical molecules became more diversified structurally, but unfortunately not pharmacologically.
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