Answer:
because strong heated makes temperatures and pressures reach the substance's triple point in its phase diagram, So the substance can be in a liquid case , and that is against the sublimation process meaning
which define as the transition of a matter or a substance from the solid phase to the gas phase directly, without passing through the intermediate liquid phase
sublimation process happens by absorbing of heat which makes molecules overcome attractive force between each other and changing to gas phase
but in the previous case it will pass through the liquid phase so it will not be sublimation process any more