It was a sales technique employed by Charles Taze Russell. Russell published his teachings and interpretations of the Bible. He marketed them as being better than reading the Bible because he was literally the only person on Earth who could explain it all as he was the Faithful and Discreet Slave.
This marketing technique continued after Russell's death with Rutherford. All that changed was that the number of people expanded from one to eight. Now the Governing Body tells everyone only they can explain the scriptures and no one should do their own research.