I believe it is because Rachel, being the wife that Jacob truly loved --not Leah the wife he was tricked into taking-- is considered in Jewish tradition, after Rebecca, as the mother of the nation. Just as on the patriarchal side, Israel historically venerated Abraham, Isaac, and Jacob (and still do);.on the matriarchal side they venerated Sara, Rebecca and Rachel.
Jeremiah 31:15 was given at a time when the Jews were being marched into captivity and says that Rachel was weeping over her children in Ramah. Why Ramah? Because it was from there that the Babylonians were marching them into captivity and it was also there that Rachel died in childbirth (of Benjamin). But she was on the road from Bethel to Bethlehem when she died and her face was toward the destination where her beloved was taking her. And as she died she wept over the son who took her life. She named him Ben-oni, which means son of my sorrow or pain. Later, Jacob renamed him Ben-jamin, which means son of my right (hand).
Both events taking place near the same place, Jeremiah takes the original significant event and uses it to symbolize the mourning over the latter event-- the Babylonian victory and the resulting Jewish captivity. But God takes it and uses it prophetically to refer to the mass murder.of the baby boys in Bethlehem...the destination to which Rachel was looking when she wept and died. It's as though, even long after her death, Rachel in eternity is still looking to that city and mourning for the dead babies of the nation that sprang from the loins of her beloved.
Does that help at all?