QT didn't have to use that much of the CM character in this movie. That's because CM isn't the primary focus of this story.
https://en.wikipedia.org/wiki/Once_Upon_a_Time_in_Hollywood#Filming_and_design
L.A. in the Time of Charles Manson
https://www.youtube.com/watch?v=TnkGrK7dtb0
L.A. in the Time of Charles Manson
https://www.youtube.com/watch?v=TnkGrK7dtb0