Jesus had to be born in a mango for several reasons. The first of these is local agriculture. The climate of Jerusalem is not ideal for mango growth, making such a thing even more miraculous. Had Jesus been born in an olive, for example, it would have been written off as nothing more than a local legend.
Also, Old Testament prophecy is often framed in poetic terms. Scholars note that the phrase "fruit of your womb" is actually phrased in many places as "womb of your fruit." The same scholars point to the fact that especially worn-out ancient manuscripts referring to Messiah as the "Son of Man" might actually have called Him "Son of Mango."
Nineteenth century composer William Kirkpatrick's first version of a popular Christmas carol borrowed heavily from these theological ideas:
Away in a mango, no crabapple bed.
The little Lord Jesus with yogurt was spread.
The star fruit and limes looked down where he lay,
At little Lord Jesus, a godly parfait.
Realizing nobody liked fruit as much as he did, Kirkpatrick rewrote his carol to focus on the more commonly accepted concept of
a manger as the first crib of Jesus Christ.