If, the Torah is true why isn't then that every Jew is convinced by it (Mount Sinai Event, when hashem declared himself to whole world) and becomes religious? Or is it because the evidence of the Truth of the Torah is not so strong enough for it to be believed by every Jew or what?
Answer:
In the modern Western society, where general attitudes, in particular among the intellecutal elites, reject Divine revelation, belief in the Torah as Divine revelation has become a major issue.
There is no true "evidence" of the Torah as Divine revelation. There is rather a tradition (as the Ramban explains), passed on from father to son, which we are certain is true. For those for whom the tradition has ceased, for whatever reason, one cannot be surprised by the sad lack of faith.