TLDR: 60% DNA shared with humans + 4 chromesomes for easy mutation tracking + 2 week reproductive cycle for fast generation studying + easy to house/keep alive = great lab specimens to test genetic mutations
But, unless this has changed recently, impossible to preserve without housing them and keeping them alive. You can freeze cell lines, nematode worms, and vertebrate sperm and eggs, but there's no analogous technique for fruitflies. Once you've made a useful mutant, if you want to keep it, some poor graduate student is going to have to keep flipping those flies forever.
That 60% shared DNA metric is rather misleading. But, yeah, flies are an amazing tool for studying biological processes, from the simple to the complex.
It's striking how useful they were when we really new very little about how the information of life was encoded and how they continue to be a powerful and relevant model system for studying more complex aspects of biology, such as the brain e.g. https://www.janelia.org/lab/rubin-lab .
But, unless this has changed recently, impossible to preserve without housing them and keeping them alive. You can freeze cell lines, nematode worms, and vertebrate sperm and eggs, but there's no analogous technique for fruitflies. Once you've made a useful mutant, if you want to keep it, some poor graduate student is going to have to keep flipping those flies forever.