What would it take for a TV show set in space to become as big as Game of Thrones?

[VampyreWraith]

In 1966, I don't think that appealed to a significant fraction of the target audience.

Now, if they had Yeoman Rand topless, you might hit pay dirt with that.

Women watched TV in the 60's, I'm sure they hoped shirtless Kirk would draw some in, even if it was a sci-fi show and they didn't think women watched those. He had a tear away shirt, almost every time he got into a fight his shirt would get ripped off. That had to be to try to get the ladies watching lol. And while the women may not have been topless, most of their uniforms were impractically short, I have shirts longer than those uniform dresses.

[BruTak]

Mr. Spock had more female fans than Capt. Kirk.

There was just something about Leonard Nimoy's looks and Spock's personality that pushed the buttons of certain women.

[VampyreWraith]

Lol, I know, I prefer Spock to Kirk too (though I wasn't born yet when it originally aired), but that probably wasn't what the network execs thought would happen lol. Spock is probably my second favorite Trek character (after Picard). Kirk is not one of my favorites.

[BruTak]

But to return to the question at hand.

Traditionally, sci-fi is very expensive to make.

To use Doctor Who as an example - you have one standing set (the TARDIS console room).
One main cast member (the Doctor) and anything up to three regular supporting cast members (the companions).
You're in a different location every episode, a different time period every episode, a different guest cast every episode.
You're filming on location a lot of the time, location work is also very expensive.

It's not like a remake of something by Charlotte Bronte, where there's a large cast, the costumes can be hired from a specialist company, there's plenty of stately homes up and down the UK to film in.