In a word, Microorganisms.
Trees do not heal or regrow new tissue when they have a wound. They actually compartmentalize (kill off) the area and clog the walls to prevent the spread of microorganisms. This area becomes dead and decays over time.
Microorganisms pioneer wounded trees and a freeze damaged Eucalyptus or Willow Acacia is a prime candidate. If you are able to see the obvious wounds then the tree is most likely fighting a loosing battle and in time the microorganisms will win.
What can you do? Well do not trim the tree. Trees form a callous (wall 4) its final layer of defense to stop the spread of microorganisms. If you cut the dead or decaying wood from your tree you most likely will cut through the 4th wall and kill the tree. Spread of microorganisms is rapid at this point being allowed into the trees feed tubes.
These are the main reasons why your Freeze Damaged and Dead Eucalyptus or Willow Acacia will not grow back. For more information call a Tree Service Company specializing in Removal.