The simple, though not very useful, answer is 'because it is not a turn - it's a corte'
Why is it not a heel turn?
Because it does not meet the definition or use the generally recognised technique.
Guy Howard's definition of a heel turn is "The backwards action of an open turn when the partner stays in line". The corte is not an open turn so it fails this test.
The ISTD definition is different and at first glance may appear to apply to the corte: "A turn that is first commenced on the ball of the stepping foot and then continued on the heel, the closing foot being kept parallel throughout." However, it goes on to say "The weight is transferred on to foot that has closed as the turn is completed."
Simplistically, we go into a heel turn by stepping backwards on either foot, and we come out by stepping forward on the same foot. So for a reverse turn we enter backwards on the right foot and exit forwards on the right foot.
For the reverse corte, we enter by stepping backwards on the right foot but we exit by stepping backwards on the left foot. Completely different.