Why does Quran not address people other than Jews, Christians and Muslims?
Quran claims to be an open book for those who want to have true guidance. But in its ayaat it usually addresses Jews, Christians (the Nisara) and Muslims only. Does Islam not consider other religions like Buddhism, Hinduism etc. important enough? Or does it consider them as sub-religions of Judaism, Islam and Christianity. Or Islam does not want (or require) to address those religions in the Quran? Please clarify.
My understanding is that it is wrong to expect from the Qur'an that it would comment on each and every ideology of the world that was in existence at the time of its revelation. Although the Qur'an is a book of guidance from the Almighty for all times to come, He has talked in it about the issues that were directly relevant to the immediate environment of prophet Muhammad, alaihissalaam. The basic purpose and the direct subject matter of Qur'an was the successful completion of prophet Muhammad's worldly mission of ensuring the earthly dominance of the ideology brought by him in the Arabian peninsula: "It is He Who has sent His Apostle with Guidance and the Religion of Truth that he may proclaim it over all religion even though the Pagans (al-Mushrikun) may detest it." (61:9) We therefore find that the religious problems associated with the people who were his immediate addressees have been directly discussed in the Qur'an.
The Jews and Nasaara (Christians settled in the then Arabia) have been talked of in detail in it because these two religious communities were directly confronted by the prophet. We don't find the mention in the Qur'an of the beliefs that were not held by the Christians of Arabia, even though they were very much found in other Christians of the world. For instance, we don't find in the Qur'an the mention of the belief that Jesus, alaihissalaam, died at the cross for the sins of mankind. Likewise, the polytheists of the Arabian peninsula have been directly addressed in the Qur'an, while Hindus, Buddhists and other religious groups have not been directly addressed for the same reason.
It would have been against the interests of the immediate purpose of the Qur'anic mission of enabling the religion of Allah to prevail over all other religions of the Arabian peninsula had the Qur'an talked about the other religions of the world theoretically. It was the duty of the Muslims of the later times to derive the understanding from the Qur'anic verses to find correct approach towards other religions too in the light of what the Qur'an has mentioned regarding the people of the Arabian society. Indeed there is enough material available in the contents of the Qur'an to enable Muslims to form correct opinions about essentially all the important religious matters man confronts even today. As for the detailed solutions on these matters, the Almighty wants us to use our intellect which has been given to us by Him to find out solutions to them.