Why wasn't Rachael buried with Jacob but Leah was?
Rachael died shortly after childbirth during a journey from Bethel to "The Tower of the Flock", which is believed to have been in the area Bethlehem. "Then they journeyed from Bethel. And when there was but a little distance to go to Ephrath, Rachel labored in childbirth, and she had hard labor. ... So Rachel died and was buried on the way to Ephrath (that is, Bethlehem)" (Genesis 35:16, 19). Techniques for preserving bodies was not practiced by nomads, so burial was usually done quickly. Even though from a map we see that the family burial ground in the cave of Machpelah, near Hebron was not that far away, it was still a long journey for a very large family with large flocks -- too long to be carting an unpreserved body. It is also very likely that Jacob was too grief stricken to do much more than have his beloved wife buried where they were.
"Then he charged them and said to them: "I am to be gathered to my people; bury me with my fathers in the cave that is in the field of Ephron the Hittite, in the cave that is in the field of Machpelah, which is before Mamre in the land of Canaan, which Abraham bought with the field of Ephron the Hittite as a possession for a burial place. There they buried Abraham and Sarah his wife, there they buried Isaac and Rebekah his wife, and there I buried Leah" (Genesis 49:29-31).
We don't know when Leah died, but at her death Jacob did entomb her in the family burial cave. In Egypt, the art of preserving bodies was practiced (Genesis 50:2) and Jacob as a dying wish stated that he wanted his body placed in the family burial cave.
If there was more to it than these simple facts, the Bible doesn't tell us. We do know that it bothered Jacob because he mentioned it in his old age at the blessing of Joseph's sons. "But as for me, when I came from Padan, Rachel died beside me in the land of Canaan on the way, when there was but a little distance to go to Ephrath; and I buried her there on the way to Ephrath (that is, Bethlehem)" (Genesis 48:7).