Here's why:

* Tissue culture is the technique of growing plant cells, tissues, or organs on a nutrient medium under controlled conditions.

* Micropropagation is a specific type of tissue culture that uses small pieces of plant material (explants) to produce large numbers of genetically identical plants. This is achieved by inducing the explants to form callus (undifferentiated tissue), which is then stimulated to develop roots and shoots.

Essentially, micropropagation is a specific application of tissue culture techniques. It's like saying "driving a car" is a method of "transportation".