what was the reason for that?
Spielberg  felt it would do nothing but rob the original of its virginity.
E.T. the Extra-Terrestrial
 Spielberg decided against pursuing it, feeling it "would do nothing but rob the original of its virginity. "E.T." is not about going back to the planet".
which actress contributed to developing the voice of e.t. the extra-terrestria Pat Welsh are there any sequels of it in the short film genre? Yes, "A Holiday Reunion" was there any sequel that got canceled? Yes, "E.T. II: Nocturnal Fears"
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E.T. the Extra-Terrestrial [SEP] Cancelled feature length sequel
In July 1982, during the film's first theatrical run, Spielberg and Mathison wrote a treatment for a sequel to be titled "E.T. II: Nocturnal Fears". It would have shown Elliott and his friends getting kidnapped by evil aliens, and attempting to contact E.T. for help. Spielberg decided against pursuing it, feeling it "would do nothing but rob the original of its virginity. "E.T." is not about going back to the planet".