He might have been waiting to see if the jury voted to not convict based on the (claimed anyway, I haven't read the statute) failure to prove each element of the crime.
Seeing that they instead voted to convict, apparently in error, he reversed their decision per the motion.
Juries can nullify bad law (in theory) by refusing to convict, but they can't convict in absence of the elements required to constitute the indicted offense. If the DA for whatever reason didn't also indict for a "lesser included offense" that did meet all the facts, that's on the prosecutor.
If the law is just badly written, that's on the legislature, similar to many old statutes that state that "burglary" can only happen at night as that is explicitly part of the elements of the crime.