The WTS preaches that there is a single visible organization because that way, they can insert themselves between God and humanity. Being a gatekeeper in that sense confers immense power and influence on a person or group. The leadership of the WTS has always pushed the idea that he/they are God's conduit on Earth.
It's also nonsense, of course. If I can reach out to God directly in prayer, and God listens to prayers and answers prayers... why do I need a man or a group of men to intervene? Didn't Jesus already intervene in the way that actually mattered? Didn't He specifically mention that He was the conduit, not some hucksters who wouldn't even be born for some 1,900 years?
If God is within reach to me, why would I depend on someone else to act as a middle man? Too many religions treat God as if he was a CEO, needing intermediaries at every step of the way, keeping the little people from ever even catching a glimpse of him. No no no! I have a direct line. Step aside, please.