If reaching people is the real goal, then there are many many ways of reaching the greatest number of people. It would require the society to spend money....which they don't like to do. Instead, the "preaching work" keeps members very busy, and makes them feel like they are part of something important. It helps to control them because they give up significant portions of their time to do it.
Excellent points there coffee_black.
Also the door to door preaching has become the org's schtick and their corporate identity if you will and they would be reluctant to give it up to any degree however unproductive it becomes.