xylem transports sap upwards only whereas phloem transports food in both directions. why do you think there is a difference ?

A tissue called Xylem acts as a pipeline and take water and minerals from the roots up the stem, and to the leaves for photosynthesis. So xylem transports water and minerals in upward direction only.

Special types of tissues called phloem carry the prepared food which has to be transported in all directions. So phloem transports prepared food in all directions.