Why Isn't mineral transport in xylem multi-directional?
Movement through the xylem is powered by transpiration of water through stomata in leaves and a sustained tension in the xylem itself due to water's cohesive properties.  See: https://en.wikipedia.org/wiki/Xylem#Cohesion-tension_theory

The "multi-directional" transport you are talking about does not actually occur in the xylem.  Minerals have to be transported OUT of the xylem before they can move other ways.  Transport THROUGH the xylem is uni-directional; transport OUTSIDE of the xylem can be multi-directional.