I used to date a biologist who experimented with animals, primarily zebrafish. And she said they chose these creatures for quite a few reasons:

1. When genetic mapping was expensive, they were some of the first mapped.

2. They procreate quickly. Faster reproduction cycles means quicker experiments.

3. Because they were some of the first, more biologists have more experiments with these genomes. So faster and more advanced work can commence.

4. Nobody complains about experiments with what effectively amounts to insects and minnows (yes, I know zebrafish aren't - im going after the sentiment here).