I can’t’ answer for the JWs; but the best answer from a Mormon perspective is to put yourself in the situation of someone asking that question in the early days of Islam. Why would someone in the early days of Islam, or in the days of the Prophet Muhammad, want to be a Muslim? It had no “spiritual tradition” as yet. It was a young religion. Spiritual traditions take time to develop. Your question amounts to saying, “A religion is not worth joining until it is old!” Well that is silly. If a religion is true, it is worth joining at any stage in its history. If it is not true, then it is not worth joining at any stage in its history.