I asked a dub this question, and have heard similar things in the past. If I asked a dub why they believe in the divine appointment of the watchtower as Jehovah's one and only organization I think they would answer in the following way.
1. Because of the success of the org, and all the info they put out, and the preaching work. It has to be divinely directed, there is no other explanation.
2. Because of how well what the org teaches matches up with the Bible.
3. There has to be one true religion! If it is not the jw, then who is it? Nobody even comes close to showing the traits we should look for in Jehovah's one true organization.
4. Because the jw is the only religion that preaches the good news of the kingdom, which is a real government.
Any others?