New theories are rarely accepted by the medical establishment unless they have been 'proved' through animal experiments. Animal research is flexible - it can be used to 'prove' almost anything. All you have to do is select the appropriate species. Animal toxicity 'tests' provide a legal defence for drug manufacturers, when humans suffer adverse effects, the company can claim to have adhered to 'safety' tests when confronted with law suits. Animal research is publishable.
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.