I'm going to wager a guess at this...
I think it must be more of a time/money saver than anything else. Gone are the days when a single judge presided over all of the cases. It would require more judges to be hired and more time to go through all of the cases. Grand Jury's make sense for this purpose.
From your post I am not sure if you know what the purpose of the grand jury is. It does not take the place of a trial or a trial judge, but rather is a process that has long been a part of the judicial process to assure that there is enough evidence for the prosecution to proceed to trial.
Stan, think about all the changes in our lives since the 60s. The widespread use of drugs alone accounts for more cases being filed not only for the possession and sale, but also for the criminal acts that users commit to obtain funds for the purchase of drugs.
We also may have been more naive in the 60s, or at least a bit more sheltered by the adults back then. Did the paper even publish the grand jury activities then, and did we, as youngsters then, care.