Fly meat is likely a more efficient source of protein than traditional livestock, such as cows, sheep, pigs, chickens, and the like, in that:
- Flies can feed on essentially any biomatter, including biomatter not edible by traditional livestock - in some cases, that means literal shit.
- While I'm unsure as to whether this applies to flies, crickets are ~40 times more mass-efficient in terms of unit mass of protein produced per unit mass of water consumed. Less water consumption equates to lower costs and improved environmental friendliness.
- From the same source as the one immediately prior to this, cricket meat is ~69% protein, while beef is 29% protein - i.e. more protein per unit of mass, which improves efficiency in more ways than I can list. I don't think it's unreasonable to presume that this might apply to flies, although I can't find a source on this.
- Insects in general produce fewer greenhouse gases than conventional livestock, reducing the effects of climate change due to farming.
- Flies have short lifespans and reproduce quickly, meaning that, if genetically engineered/bred into being food insects, such modifications take effect much faster, and the effects of such modifications can be rapidly observed. Basically, they have a fast turnaround time in terms of genetics.
- People don't care about flies. All but the most stringent animal-rights activists and moralists do not care about their lives. PETA does not protest fly deaths, nor would they compare fly factory farms to the Holocaust. You will not be facing essentially any opposition to a fly farm on the animal-rights front.
- Flies live everywhere, meaning that they can be bred anywhere - all you need is an enclosed space for doing so, so that they don't fly away.
However, there are obstacles to fly farming. Ignoring any cultural ones - for instance, people not wanting to eat protein sourced from such stereotypically and often legitimately filthy insects - but taking into account any other potential issues with this: how, exactly, does one farm flies on an industrial scale?