One wonders if it isn't his personal preference. "Just call me Mister. 'Sir' reminds me of my father..."
In "The Cage", the form of address is suitable, because Spock is a junior officer. He even calls himself
Mister, perhaps not being all that fluent in Earthlingese...?
Yet in the very next one, "Corbomite Maneuver", the junior Sulu readily refers to his superior as Mister Spock already. Perhaps the early writers were expected to read the script for "The Cage" and derive their cues from there, and they either weren't all that well versed in militaria, or believed that this was some sort of a futuristic scifi thing?