Because there were 200,000+ prophets. And no you cannot conclude that no prophet was sent to these people because quran speaks truth.
And for every Ummah (a community or a nation), there is a Messenger; when their Messenger comes, the matter will be judged between them with justice, and they will not be wronged. (Qur'ân 10:47)
And verily, We have sent among every Ummah (community, nation) a Messenger (proclaiming): "Worship Allah (Alone), and avoid (or keep away from) Taghut (all false deities, etc. i.e., do not worship Taghut besides Allah)." Then of them were some whom Allah guided and of them were some upon whom the straying was justified. So travel through the land and see what was the end of those who denied (the truth). (Qur'ân 16:36)