Here's the reason:
"Thus the Bible is an organizational book and belongs to the Christian congregation as an organization, not to individuals, regardless of how sincerely they may believe that they can interpret the Bible. For this reason the Bible cannot be properly understood without Jehovah’s visible organization in mind." (The Watchtower; 10/1/1967; pp. 587)
"We have the opportunity to show love for our brothers who take the lead in the congregation or in connection with Jehovah’s visible organization worldwide. This includes being loyal to "the faithful and discreet slave." (Matthew 24:45-47) Let us face the fact that no matter how much Bible reading we have done, we would never have learned the truth on our own." (The Watchtower; 12/1/1990; pp. 19)
*** w81 2/15 p. 17 Do We Need Help to Understand the Bible? ***
From such experiences it can be seen that Jehovah God caused the Bible to be written in such a way that one needs to come in touch with His human channel before one can fully and accurately understand it. True, we need the help of God’s holy spirit, but its help also comes to us primarily by association with the channel Jehovah God sees fit to use.—1 Cor. 2:6-10.
Eden