You need the core to have a high permeability for magnetic flux. This ensures that all the magnetic flux lines are channeled through the core and not through the dielectric. read more
Aluminium doesn't have these magnetic properties hence aluminium is not used , aluminium is paramagnetic material ,it has good conductivity which is not needed in transformer core since then Eddy current losses will be more . read more
Since the resistivity of copper is 0.6 times that of aluminium, the cross-section of the aluminium conductor needs to be 1.66 times larger than that of the copper conductor for the same resistance. This results in a larger transformer core and volume, which also leads to a larger transformer tank than for the copper design. read more