To not let people be aware so that everyone can be saved from hell defeats the purpose of free will. Why then would god have created the earth if everyone was to go to heaven? One chooses whether they want to believe in God or not, act according to their own conscience until the Day of Judgement.
51:56:
“I have not created men except that they should serve Me”
23:115:
“Did you think that We had created you in play (without any purpose) and that you would not be brought back to us?”
God created man to serve Him, meaning that men should believe in the One Lord and do well. This is the main objective of human life. A test.
17:15:
“We do not punish [any community] until We have sent [it] an apostle.”
This means that those who have never had a full authentic hearing of the religion cannot be blamed, one might say, for their “invincible ignorance” (catholic term). Today, these tribes that you are talking about, we can both agree, comprise a minute population of the human population. Consequently they are not punished. And while they might not be aware today or tomorrow, a time will come where they will become exposed to the outer world as humanity continues with its advancements.
God can make anything happen, but that defeats the purpose of free will and so to follow, the concept of heaven and hell.