well the argument is that if the state is so inconpetent that it keeps mistrying an individual, then that's potentially harrasment of an innosent person and can't be allowed
Given the role of a jury as ground truth to the legal system, it's rather dangerous to hold that a jury can "misbehave".
Jury nullification, as controversial as it is, is an important escape valve and check on the system. Prosecutors and judges just don't like it because it threatens stare decisis.