Why didn't Jacob bury Rachel in Machpelah?

The cave of Machpelah was in Hebron.

Ge 35:

16 Then they journeyed from Bethel. When they were still some distance from Ephrath, Rachel went into labor, and she had hard labor. 17 And when her labor was at its hardest, the midwife said to her, “Do not fear, for you have another son.” 18 And as her soul was departing (for she was dying), she called his name Ben-oni; but his father called him Benjamin. 19 So Rachel died, and she was buried on the way to Ephrath (that is, Bethlehem), 20 and Jacob set up a pillar over her tomb. It is the pillar of Rachel’s tomb, which is there to this day. 21Israel journeyed on and pitched his tent beyond the tower of Eder.

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Bethlehem was 40 km away from Hebron. It would have taken 2 days to get there. Jacob buried Rachel on the way to Bethlehem out of practical considerations. Much later:

27 Jacob came to his father Isaac at Mamre, or Kiriath-arba (that is, Hebron), where Abraham and Isaac had sojourned. 28 Now the days of Isaac were 180 years. 29 And Isaac breathed his last, and he died and was gathered to his people, old and full of days. And his sons Esau and Jacob buried him.

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They buried Isaac in Machpelah.

Moreover, Ge 49:

31 there they buried Abraham and Sarah his wife; there they buried Isaac and Rebekah his wife; and there I buried Leah.

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Joseph buried Jacob in Machpelah (Ge 50:12). All three pairs were buried in Machpelah.

Why didn't Jacob bury Rachel in Machpelah?

Rachel died of a sudden death while they were traveling near Bethlehem. It was expedient to bury her right there rather than taking the dead body over the next two days to bury her in Machpelah.

There was also a spiritual dimension to this. Jeremiah prophesied in 11:

15 Thus says the Lord: “A voice is heard in Ramah, lamentation and bitter weeping. Rachel is weeping for her children; she refuses to be comforted for her children, because they are no more.”

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Ramah symbolized Bethlehem.

Herod killed the innocent children in Mt 2:

16 Herod, when he saw that he had been tricked by the wise men, became furious, and he sent and killed all the male children in Bethlehem and in all that region who were two years old or under, according to the time that he had ascertained from the wise men. 17 Then was fulfilled what was spoken by the prophet Jeremiah:
18 “A voice was heard in Ramah, weeping and loud lamentation, Rachel weeping for her children; she refused to be comforted, because they are no more.”

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Why didn't Jacob bury Rachel in Machpelah?

Horizontally speaking, it was due to the practical necessity to bury her right away near Bethlehem. Vertically, God used it to prophesy Herod's massacre of the innocent children in Bethlehem after Jesus was born there.