There is limited space and time in a motion picture. You need to be economical with your music, not just for the normal reasons.
Half the time when you have a film cue, something else is happening in the movie. Either there is action on the screen or someone is talking, etc.
The human brain has a limit to what it can process. The more notes you have, the less processing time for watching the movie.
When you write a song, you want to keep the audience completely engaged in your music. You want lots of notes and lots of instruments.
But this is NOT true in a motion picture. Most of the time, you want to create a mood without the audience having to think about it.
When you win an Oscar for best film score, you don't want the audience to say, "Hey, that was great music!!!". Instead, you want the audience to say, "Music? I didn't know there was any music in that movie."
If your music requires that the audience spend all its time concentrating
on your music, the director and the producer of the motion picture are going
to get mad. Their solution is to TURN DOWN THE VOLUME of your music.
So, to make everyone happy, PLEASE use as few a notes a possible.