A primeira coisa que vem à mente sobre o porquê disso foi o fim do ROTJ.
VADER
You cannot hide forever, Luke.
LUKE
I will not fight you.
VADER
Give yourself to the dark side. It is the only way you can save your
friends. Yes, your
thoughts betray you. Your feelings for them are strong. Especially
for...
Vader stops and senses something. Luke shuts his eyes tightly, in
anguish.
VADER
Sister! So...you have a twin sister. Your feelings have now betrayed
her, too. Obi-Wan
was wise to hide her from me. Now his failure is complete. If you will
not turn to the dark
side, then perhaps she will.
LUKE
Never-r-r!
Luke ignites his lightsaber and screams in anger, rushing at hisfather
with a frenzy we have not seen before. Sparks fly as Luke and Vader
fight in the cramped area. Luke's hatred forces Vader to retreat out of
the low area and across a bridge overlooking a vast elevator shaft.
Each stroke of Luke's sword drives his father further toward defeat.
The Dark Lord is knocked to his knees, and as he raises his sword to
block another onslaught, Luke slashes Vader's right hand off at the
wrist, causing metal and electronic parts to fly from the mechanical
stump. Vader's sword clatters uselessly away, over the
edge of the platform and into the bottomless shaft below. Luke moves
over Vader and holds the blade of his sword to the Dark Lord's throat.
The Emperor watches with uncontrollable, pleased agitation.
Poderia ser menor, mas Vader usou essa informação para instigar Luke em ação finalmente.
Além disso, se você acredita no que Lucas diz ser verdade sobre ter todo o enredo feito na maior parte, mesmo quando os 3 originais começaram, então Leia tendo a força foi essencial para o enredo das histórias 7-9.