They are the ones making the claim, they should be the ones to prove it. Its like someone asking: Prove that a tea pot is not orbiting around the earth. Wouldn't it make sense for the burden of proof to be placed on the one making the claim in the first place?
Nevertheless, here are some reasons I can think of :
- JWs don't practice forgiveness in a Christian manner. For instance, a women is disfellowshiped for fornication; then marries to fix the situation. She will remain disfellowshipped until she follows several man made rules. In the meantime, she will be treated the same as someone God hates. Logically, the moment she says "I DO" should be it.
- They judge people excessively. They will shun people for smoking, voting, celebrating birthdays. If they stop to think about Jesus' teachings, it is excessive.
- They made prophecies that have not come to be true. They try to weasel they're way out of it playing with words, but the bottom line is: They made assertions claiming to be God's teachings and they failed.
- They conceal and lie about their history. Anyone who talks a bit too long on their troubled past and teachings will be considered apostates or spiritually weak at best.
- They teach that God, who loved humanity so much that he gave his only son, will soon come to destroy its greater majority, a majority who never heard their message!
- A group of man demand the same authority as Mosses and other prophets in the past had while claiming NOT to be inspired. You cannot have it both ways.