Your question is a very good one.
Watchtower
“Down through the years the slave-like congregation has been feeding its true members faithfully and discreetly. From Pentecost, AD 33, up to the very present hour this has been lovingly and carefully performed. Yes, and these domestics have been fed on progressive spiritual food that keeps them abreast of the “bright light that is getting lighter and lighter until the day is firmly established”. (Prov 4:18) All this has proved to be “food at the proper time” as stated by Jesus.”
So the claim is that this faithful & discreet slave was to have a continuous uninterrupted history down through the centuries up to and including the end of the world. The congregation, through this spiritual food would become increasingly enlightened. Each century & succeeding generation passing on increased illumination. By 1870 & CT Russell, the light should have been very bright, so why did he not receive guidance from this close-knit organised & enlightened body of Christians. After all the WT says “we cannot find the Scriptural guidance outside the faithful & discreet slave organisation.”