I think the Grand Jury was called off to keep them alive with the planned violence by demonstrators

 So, it is a self defense position to put it off for another day in the hopes that demonstrators who might become violent like in January 6th insurrection might financially have to go home instead of demonstrating. I personally think this is very wise, especially for the New York Police Force to better prepare for eventual violence (whether it happens or not) when they arrest Trump soon after the Grand Jury deliberates.

But, the only slam dunk is for the misdemeanor conviction which will give 1 year in jail for Trump and not the felony because it will be extremely difficult to prove intent which is what you will need to convict on the felony. If he is convicted for the Felony he could get 4 years in Jail.