David Gran, in 1998 in The New Republic, reprinted online by PBS:

Whereas most prosecutors must discriminate between cases–to decide, say, whether to spend their time and money pursuing people who drive over the speed limit or mug old ladies–independent counsels have only one case, and nearly all the time and the money they need to pursue it. There is little incentive to stop investigating. And, as the investigation racks up costs, the pressure inevitably mounts to convict.