Animal research is fast. Human diseases generally span many years, but laboratory animals, with shorter lifespans, tend to have more rapidly progressive disease processes. This facilitates quick publication of research projects. Many scientists are trained in and comfortable with animal research techniques. They are reluctant to adopt alternative methodologies, such as tissue cultures, which would require extensive re-training.
Animal research is controllable.
Scientists can control research variables more tightly in animal experiments than in human clinical research. The control, however, results in an unnatural and sometimes pathological setting for the animals, far from their natural habitat. The resulting stress influences the function of virtually every organ, and this confounds the data. Although controlling variables is desirable, it does not justify the use of poorly conceived animal models. Animal research is traditional.