It's because the specific specie they target isn't even native to that place (it was introduced by humans), and also represent a small fraction of all musquitos in the area.
So, we messed with the balance before and it went badly, ... that's not logically a sufficient reason to mess with the balance in the eco-system again expecting unmitigated benefits.
When one gets stabbed withdrawing the knife (which follows the same logic) can kill you.
As for small fractions, ecological systems are usually pretty chaotic (in the mathematical sense of small perturbations being able to produce massive changes).
When one gets stabbed withdrawing the knife (which follows the same logic) can kill you.
As for small fractions, ecological systems are usually pretty chaotic (in the mathematical sense of small perturbations being able to produce massive changes).