Mormon evokes the ancestors of Lehi. This is a tradition that has crossed the ocean, and has such ancient ties that they run deep into a time before Lehites were in the New world. As part of the invocation of the ancient, Mormon speaks of the blessing that have come through the ancient lineage of Jacob and Joseph (the lineage of Lehi).
There was a specific promise made to the Lehites, and that is the foundational promise, which Mormon restates here. It is quite fitting that he should bring the promise to the conscious present of his narration since he is currently involved in the dramatic demonstration of the promise and the blessings that come from righteousness that are being contrasted with the disasters that come from disobedience.