It seems so obvious to us now because we're used to the idea of screen sizes and resolutions both continually improving. But in the 1960's, when even the idea of doing television in color was a new thing, there is absolutely no way the producers could have been expected to fathom that their show would be of interest to people 50 years later, let alone what kind of display technology we would have.
Quite simply, they shot at the level of quality they needed to shoot in order to make it look good for the TV's at the time. And I'm sure it worked quite well. There's just no way you're going to take something that was shot back then for TV's of the time, look at it on a modern big screen high def TV, and not see problems with it.
That said, I think Trek overall holds up remarkably well.
Quite simply, they shot at the level of quality they needed to shoot in order to make it look good for the TV's at the time. And I'm sure it worked quite well. There's just no way you're going to take something that was shot back then for TV's of the time, look at it on a modern big screen high def TV, and not see problems with it.
That said, I think Trek overall holds up remarkably well.