|Why are doctrines like heaven and hell not mentioned in the OT?|
A common question that has been asked is, Why are the doctrines of the afterlife missing from the OT, but found in the NT so much? Frequently the simple answer is "progressive revelation" and while that is fair to say in sum, much more can be said that expands on that answer.
A simple answer to begin is that in the OT, there just isn't any place to expound on these doctrines. That's a sort of an odd answer, yes, but in the end, there is no place in a law code, or a list of proverbs, or a narrative history, or in prophetic warnings about the destruction of a city, for comments on the afterlife. If they appear at all, they should appear incidentally and without details. We have no OT equal to a Book of the Dead nor any book that would have accommodated it.
Which leads to the obvious question, Why wasn't there such a book? Didn't God care if the Jews were not warned about hell?
The question itself seems oriented towards the idea that modern "you're going to hell" preaching was also the norm in the time of the Bible, but one will note that NONE of the missionary speeches in Acts made such an appeal. Hell as an evangelistic "tool" is a more modern phenomenon, more the province of Wigglesworth and Edwards than of Jesus and Paul.
Hell in the NT is not used as a threat to convert, but noted as a matter of fact that is taken for granted, and as the eventual place of evildoers (though the evildoers themselves are not the "audience" addressed; for example Jesus' warnings to Sidon, Capernaum, and the Pharisees, were not obviously given TO them in person).
Jesus mentions hell the most in the NT, but even he says little (the word is found only 23 times in the KJV, and most of those are by Jesus, and include Synoptic parallels; "heaven" is mentioned over 250 times, but mostly as in the OT as the abode of God).
Likewise, there is no appeal for, "know Jesus and you'll go to heaven" -- the reward of heaven is treated even in the NT as a sort of incidental. Indeed a close reading of even NT texts shows that there were very few details given even about these doctrines at all.
Those who think there was some huge transition here are not reading the Bible, but the modern church. As we showed here, there is certainly a notion of afterlife in the OT; if details are demanded, we may remark in return that the average peasant of the OT period was far more concerned with their next meal and with protecting their family to be concerned with deeper philosophical and eternal problems; if you think not, ask yourself how many times YOU think of the subjects while working, eating, or playing -- whether you are a Christian or not.
Nevertheless the lack of a detailed doctrine in the OT is, to put it mildly, no big deal by any standard. The simple idea of good vs. evil, of loyalty to Yahweh as the choice for "life" (versus death, obviously) is more than enough to transmit to any person with an ounce of sense that such bifurcation would apply further in any afterlife situation.
To say that there was no information on halos, pitchforks, fire, or what have you, is irrelevant. Such speculations would only be worth the time of those who had the leisure for them -- which did indeed not happen until the intertestamental period, when the detailed doctrines of heaven and hell start to come to fruition. (Note particularly what we said in the link above about Sheol being a place where there is no sign the righteous go.)
Furthermore, given the present-orientation of the ancients, we would expect far, far more to be said of present circumstances than of what will happen in the future and in the afterlife.
Some argue that ideas of heaven and hell came from, for example, the Persians, or late mystery religions (!) but this posits a situation in which Israel's neighbors in Egypt had such ideas and Israel did not, which seems absurd. It is certainly absurd to suppose that there were NO ideas about the afterlife among the Jews, regardless of how much or how little the OT records, and the forbiddance of necromancy by itself suggests that a belief in some sort of afterlife was in place, even if it was not talked about it detail.
Ideas of an afterlife, and of a fate one way or the other, are simply common sense in a religious setting, and the lack of detailed explication in the OT is simply nothing to argue about.