For me when I started watching it in the 1960's the hook was the open ended nature of the concept. As others have said the nearest competitor was Lost in Space (also Land of the Giants, which I watched avidly) programmes with a single goal, to get back home, which every episode leads to hope then disappointment. Programmes like Star Trek and Doctor Who were simply exploring, they were not trying to get anywhere, they could go home (admittedly the Doctor didn't really want to) they could go on missions. It felt boundless. Coupled with alien main characters like Spock that had their own unique culture and weren't simply monster of the week made it very compelling for a young me.