SBF: Gary and Heather Bottings' ... noted the function of the habit of the Watchtower organisation of changing organisational terminology periodically. ... The Bottings argued that such changes served the function of weeding out inactive members by periodically rendering their terminology obsolete.
That's one possibility, but I think there is another more practical and far-reaching reason: Since nothing REAL is happening in fulfillment of any JW "prophecies" or "theology," they need to manufacture changes periodically to give/maintain the illusion of being progressive (you remember all that "Jehovah's Chariot is on the move" bullshit).
You may appreciate this thread I started a while back which discusses this phenomenon: