In addition to all the reasons listed above: usually by the time a case goes to trial if it hasn't been plead out or dismissed the DA has the guy dead to rights.

Like I remember one where I was a witness where the guy had called in prank bomb threats to avoid bunch of schools (i was there to testify that we'd had to evacuate the building) and like they had the guy's cell records, 3 people who could ID his voice, and a guy who rolled on him.

At the end of the day the DA struck a deal on lesser charges just to avoid a trial