The Old Testament, in many respects, is like living in the night. It's dark and difficult to see, but there is some light reflecting off the moon, so we're not completely blind. And when we live by the moonlight, although we can roughly see where we are going, there are many weird and dark shadows which can make even that little bit of light to be confusing, as we look at one thing and think that we are seeing something else altogether. But as time goes on we get closer to dawn. And as dawn advances and we get close to sunrise, we begin to see more and more of the light of the sun itself, and not just its reflection off the moon. And finally the sun appears and we are able to see the source of light and we are able to see everything for what it truly is.
In that period we did not have the light of Christ as we do today. But his light shone through the prophets and we had just enough of His light to get by. But even with that light and that revelation through the prophets, many things were still unclear, and there were some things that we couldn't see at all. But as we got closer to Christ's incarnation, His light began to be more and more clear. And as you read the Scriptures that were written much later, especially those in the couple centuries just before Christ, there is much more clarity in those writings, and certain truths that were only barely discernible at the beginning are seen in much better light.
The resurrection is one of those things. In the days of Moses we can only see glimpses of it, and Christ Himself shed light on the resurrection in the Torah when He spoke to the Sadducees. But in the very last books to be written in the OT period, we can see the resurrection much more clearly. Take a look at 2Macc 7, where discussion of the resurrection is almost as explicate as what we find in the New Testament.