It's quite simple really. During Russell's leadership, there was emphasis on Jesus, and character development. Transforming oneself into the likeness of Christ, That was of the utmost importance in the early years.
That is why in the early Watch Towers prior to 1916, you will see many articles on Jesus and overcoming. While Russell taught two salvations, one heavenly and the other earthly. He taught and believed that the Lord is calling and inviting ONLY those individuals who are willing to sacrifice all and follow Jesus with the reward of being co-heirs with Him in heaven.
There was no stressing to live on earth, the "One Call" was heaven, when the Church was complete, then the mediatorial reign would begin and the rest of humanity would live forever on earth.
Which is why the Society in Russell's day was very small, they weren't interested in numbers, only in finding those who are receptive to the message.
When Rutherford took over, he downplayed Jesus, he stated that character devlopement studies were of the devil and that the most important thing was witnessing.
Of course by then, the majority of Bible Students who were faithful to Russell defected and abandoned the Society to continue the work that Russell started.
Rutherford replaced Russell's writings with his own, and then replaced his own writings with more of his own writings, He wrote at least three books a year, dozens of booklets a year, and authored ALL Watchtowers articles, along with specialty papers and tracts. This kept those Bible Students who stayed busy, studying his writings and going door to door. The studies were no longer roundtable discussions like they were in Russell's era.
Thus as time went by, more Bible Students left, leaving pretty much those who came either a few years before Russell's death or mostly those who came after his death.
In the end, the Judge kept everyone so busy knocking on doors and studying his writings, that Jesus and character development was lost sight of, since Rutherford's message was a message of damnation, join us now or perish, armageddon is coming, babylon has fallen and will be destroyed, millions now living will never die, etc.
Those who stayed, lost sight of Jesus and his message, they were "ever learning and never coming to an accurate knowledge of the truth." The judge had distance them from their one goal, of being like Jesus, how can you be like Jesus, if you don't even know him??? Bookstudies replaced Bible Studies.
Many were feeling unworthy of heaven, the Judge turned their "New Creatures" into "salesmen."
In the early 1920s, the Judge was already toying with the idea of an earthly class, he had various names for them, jonadabs, other sheep, etc. Because of his witness endeavors, the Society has grown to more than 144,000, he had to think of something. So he turned the Great Crowd from a heavenly class as taught by Russell, into an earthly class in 1935, and the masses were relieved. Why? Because as I said, the Judge stripped them of their Christianity, they weren't spiritual.