Because it's a television classic and because it spawned an entire television and motion picture franchise, novels, comics, etc.
and I feel that TOS is more over-rated for the exact same reasons as what many have said about TOS.
It's too 60's, it's too dated.
TNG was too 80s. What's the difference? For me, that's part of ST's (TOS's) appeal. I love the Matt Jeffries Enterprise designs and the low-budget look of the show. They showed a lot of creativity and artistry despite having a much more meager budget compared to TNG's.
It's too random and all over the place.
So was TNG, VOY, and ENT.
TOS has many characters that rarely if ever get an episode about them.
Because it wasn't an ensemble show. That wasn't the style of TV at the time. You could just as well say that that what was made it better, because, honestly, I wasn't that interested in Sulu, Uhura, or Chekov. Kirk, Spock, McCoy, and Scotty are my favorites.
The series has no specific or tangible plot line like all the other series do.
TNG didn't have a specific or tangible plot line either.
It's just something random every week.
Again, like TNG.
TNG at least had some specific story lines going on each year that were focused on.
No, it didn't. They did have a bit of serialized or recurring storylines, but there were never ongoing arcs like there were on DS9 and later on ENT.
And even minor characters got attention.
Sure, they got attention, but honestly, I didn't really care if I knew more about Geordi, Crusher, Troi, etc.
In TOS the only characters who ever get any major screen time is Kirk, Spock and Mccoy. And yes those characters are great, but I would've liked to have seen a more equal series and have every character get their share of episodes.
Why? Honestly, I was fine not knowing much about Uhura, Sulu, and Chekov. The only one of the supporting characters I really love is Scotty. And it's no different from TNG, despite it being more of an ensemble show. There weren't that many episodes centered around LaForge or Crusher or Troi or even Riker. There were far more episodes about Picard, Data, and Worf because most of the fans liked these characters more than the others and the writers responded to that and wrote for them.
I tend to prefer DS9, Voyager and Enterprise the most because those 3 series IMO are the ones that are truely about something.
TOS was about something: The 5-year mission of exploration of the USS Enterprise. TNG was similar in that regard.
TOS and to a degree TNG are about the random planet of the week. DS9 is about Bajor and the Cardassian occupation and the Dominion and gets really deep.
So what's wrong with "the random planet of the week." That can be a strength, since it's easier to get new viewers. DS9's serialized storyline became an impediment to getting new viewers.
Voyager has a very set and clear goal, to get home.
So what if it had a goal? It was still treated in all ways as TNG v2. Effectively VOY = TNG.
Enterprise is also great, their goal is to learn how to survive in space and learn how to deal with intersteller travel.
And then later to deal with the Xindi and the Klingons and the Romulans.
With the exception of the Xindi reference, you could say the same for TOS.