This is called prosecutorial discretion and in the U.S. Legal system, the Prosecutor can choose to present any case of any crime they wish. It is not Jury Nullification, in that it doesn't overturn a law, but rather, gives the prosecutor a way to not bring certain criminal charges to court. The general guiding system is that the prosecutor believes that they cannot secure a guilty verdict with the evidence on hand (and the Prosecutor is intentially dealt the weaker hand in U.S. jurisprudence. The Prosecutor must prove that it happened in exactly the one way that makes it criminal, while the defense merely has to prove it could have happened in a way that was not criminal, but need not offer a specific explanation (there was a famous child murder case in Florida in which the defendant had made numerous contradictory statements as to the nature of her actions up untill the child's body was found at various points in the lifecycle of the case (which was in the public eye for almost four years). When it went to the jury, the conclusion was that they basically, the defense likely had a significant role to play, but the prosecution's charges could not be proven with evidence presented. Scotland actually offers a third option for this situation where the jury can find "Not Proven" in addition to "Guilty" or "Not Guilty" which basically means they tend to agree with the prosecutor, but that they cannot convict because the evidence doesn't fully support. Or as one Scotsman describe's the intent of not prove, it means "You didn't do it, and you better not do it again!".).
Because the prosecutor is a taxpayer funded office and an elected office, they are answerable to local public opinion and as such, will face scrutiny for bring a shakey case both from the people who believe the guy didn't do it, and the people who think that the prosecutor shouldn't have taken this case as they knew it was not a slam dunk and thus they were wasting taxpayer money (trials cost money) and citizens time (a jury is drawn from local election registry... so anyone who is deciding a can vote on the prosecutor's future employment).
Another reason is that once a jury is empaneled, the Prosecution doesn't get a do over. The Constitution Bans double jeopardy for criminal cases, which effectively means that if new evidence is uncovered after a not guilty verdict, then the defense got away with the crime (essentially, it means only the Defense can initiate the appeals process in a criminal case. Given that no one would do that if new incriminating evidence is found, the Prosecution can't appeal the decision.). It's not a violation to not press the charges until more incriminating evidence is found, so long as it's under the statute of limitations for the crime, so they can play the long game.
It could also be entirely politically motivated, but if it's obviously political, then the it could backfire. Take a look at the Chicago prosecutor's office and the outrage from many people of many different political thoughts when she let off Jessie Smollett for falsely reporting a crime. It's really bad for a prosecutor when you lose the support of local police, who are essentially the guys who they represent in their cases.
Jury Nullification is a different event and not something the prosecution can do. Jury Nullification is when the jury comes to the conclusion that the defendant did in fact commit the crime but the law that was violated should not have existed in such a way that given this specific case they found guilt. In effect, it is not guilt by reason of a law we disagree with, and thus the state shouldn't have prosecuted in this case not because he didn't do it, but because it shouldn't have been a crime if he did. It doesn't necessarily mean the law is repealed or changed, but occasionally it can (for example, a man is on trial for stealing a loaf of bread to feed his family. The jury concludes that yes, he did take bread without paying for it, but that it wasn't taken for profit purposes as he had no intent to resell it and only stole that which he needed to keep himself and his dependents alive and the prosecution shouldn't have gone after this guy like it they did. There could be some case law that now theft doesn't include stealing basic necessities, or the prosector will realize the jury won't convict and let it go as pressing the matter will waste resources). In other aspects it could be that the law really was egrigious (for example, Baltimore City has a law banning ownership of parrots as pets. No one really knows why and no one gets arrested on it, but if they did, the Jury could say "this law is stupid" and let the guy go because a parrot ban serves no specific purpose would likely render the law effectively repealed. Ironically, many a bizarre law in Common Law Jurisdictions are created in this manner. The U.S. can create laws through legal preciseness. For example, under Flordia Law, an alligator counts as a deadly weapon for the purposes of the law... but this came about because someone through a baby alligator at someone and Assault with Deadly Weapon was charged and a judge ruled that the alligator was used as a weapon and they are deadly. It wasn't written by the state legislature because they had nothing better to do then argue about alligators counting as weapons.).
This is one of the reasons why judges will not allow either side to make any mention of Jury nullification during the case. It's also why, in the United States, cases likely to result in a capital punishment sentance can not have a jurist empaneled who is opposed to the death penalty (because sentancing only comes after guilt is found, the jury rarely has any say in the matter beyond the verdict and in the interest of justice, the court does not want a not guilty verdict or a hung trial based on someone's political position, not the merits of the case. In the case that the jury does affect the outcome, the question is only posed after they find the man guilty. It is illegal for Captial sentences to be mandatory, but it is legal if the sentence is found to be warranted based on aggravating and mitigating factors specific to the events of a specific case.
