Question

why material concept are called hypothesis

Answer 1

A hypothesis is some kind of assumption (speculation, roughly speaking), which may be not based on experimental data. In brief, the main difference between a hypothesis and a theory is that theory requires repeated testing. Malaria hypothesis is not actually a hypothesis, since it has been already validated. Although there are some new malaria hypotheses, which still require testing and experimental data. Probably, you are talking about one of such new hypothesis.