Plus, this was combed over with a fine one in hopes of charging President Trump previously and they realized it wasn’t a valid charge.
I'm sure Merrick Garland looked at charging Trump from every conceivable angle. If there was any possibility for a conviction, he would have gone forward. Of course, a federal case against Trump would have faced an emergency appeal to SCOTUS, where as NY can probably run out the clock in state appeals courts. They don't need this conviction to survive appeal, they just need Trump to be a "Convicted Felon!" until November.