Small animals suitable for experimental use must reproduce fast and their genes should be as similar to humans as possible. Some small animals reproduce fast but have few genes similar to humans. 
some fast-breeding small animals are not suitable for experiments.
According to relevant regulations, to obtain the National Scholarship for Poor Students, applicants must have poor families and excellent grades. Some of the scholarship recipients have very poor families, but their grades are not excellent. Obviously, the rules for assessing the National Scholarship for Poor Students have not been strictly implemented.