Ennio Morricone said in a recent interview to a Swiss newspaper (to promote his concert he held in Zurich - where he apparently fell from the podium at the end of the event) that he'd love to score more science fiction movies.
He said that he only did very few sci-fi film scores during his career, mentioning two, "Humanoid" and "The Thing". Therefore omitting "Mission to Mars" (which I think is an exceptionally good score in Morricone's oeuvre).
Morricone also found it strange that such a gifted composer like John Williams used a march in "Star Wars", guessing that he must have been asked by his employees to write one.
He said that he only did very few sci-fi film scores during his career, mentioning two, "Humanoid" and "The Thing". Therefore omitting "Mission to Mars" (which I think is an exceptionally good score in Morricone's oeuvre).
When you've scored films tallying in the hundreds, you probably forget a few every once in a while.
Also, I'm with Morricone; I too long to see him score more science-fiction.