I realize Spock was not initially intended to be the main character of Star Trek, it just seems like his importance to the series grew to the point where he became the main character.
Which was, as noted, the result of Spock being popular with the fans. Though he was already a lead and billed second in the credits from the start of the first season. He also gets some big scenes in the fourth episode (Naked Time
), is the driving force behind the plot of The Menagerie
and is very important to the plot of The Galileo Seven
. So it not like he's some secondary character pushed to the foreground like Fonzie or Urkle. Still the majority of the plots are still focused on Kirk throughout the series.