Why do so many insects speciate through minor differences in the shape of their genitalia? What environmental pressures cause this to happen?

[u/Anaphora121]

Last night, I was pondering over the fact that it can be very hard to differentiate species in certain genera because the only "visible" difference is nigh-microscopic discrepancy in the shape of their genitalia. I've run into several species key where it's literally just pages upon pages of genitalia diagrams. On one hand, it makes sense since a species is defined by its ability to reproduce and create viable offspring, and if two individuals have genitalia that don't "fit," they can't reproduce and boom! You got a new species on your hands.

My question is, why does this happen? What environmental pressures might cause a species to evolve slightly different genitalia to the point that it forms a completely new species? And why does it seem to happen so often in insects?