why more complexorganisms cannot give rise to new individuals through regeneration?

This is because many multi-cellular organisms are not simply a random collection of cells. Specialised cells are organised as tissues and tissues are organised into organs, which then have to be placed at definite positions in the body. In such a carefully organised complex situation, cell by cell division by simple fragmentation or regeneration cannot give rise to a new individual. Multi-cellular organisms, therefore need to use more complex ways of reproduction.