Why there was a need of change in classification system? 

Explanation

A need was felt for including besides gross morphology, other characteristics like cell structure, nature of wall, mode of nutrition, habitat, methods of reproduction, evolutionary relationship etc.
Classification system for the living organisms have undergone several changes over time.
Though plants and animal kingdoms have been a constant under all different system, the understanding of what groups / organisms be included under these kingdoms have been changing the number and nature of other kingdoms have also been understood differently by different scientists over time.