Spock appealed for a combination of reasons.
He was cool, calm, and collected. He was incredibly intelligent but also strong. At the same time he had a vulnerability. He didn’t quite fit in with the people around him. For introverts and nerds, it was so easy to identify with Spock.
In the episode Journey to Babel, Spock’s parents appear. We learn that Spock has a difficult relationship with his father because of the career he chose. We meet his human mother and hear her say to her son “When you were 5 years old and came home stiff-lipped, anguished … I watched you knowing that, inside, the human part of you was crying.”
Spock represented the conflicts between intellect and emotion.
His brilliant mind always came up with the perfect solutions and his Vulcan physique made him the guy you wanted next to you in a fight.
Spock was an alien that helped us understand what it meant to be human.