The small gene pool is done on purpose. The mice are supposed to be as close to clones as possible so that you can have control populations and be confident that the results weren’t affected by certain genes and mutations in the test population.
The size of the gene pool isn’t really an issue though because they can be bred however it’s required for tests. They have quite a lot of control over the genetics of those lab mice.
Testing for a cure for diabetes? They can produce mice that are almost guaranteed to develop diabetes that you can then try to cure.