Why are scientific theories modified, but seldom discarded?

A scientific theory is a well-explained idea which is established after performing the experiments which provide the evidence in support of the hypothesis.

The theory can be modified as well as can be rejected with time as the advancement in the technology can provide new facts which could be added in the existed theory or contradictory facts which could lead to the rejection of the theory.  

The generation of new data can shift the paradigm of science on the basis of which the scientific theory is based.

what is the question because that is not a question.

.) the observation by semmelweis that more mothers died of childbed fever when their doctors also worked in the morgue