Not sure why. Maybe network censorship back in the day?

No, it's technology. All the things that let us time shift when we watch TV. VCR's, cable, DVD's, streaming.

Back in the day, you could only watch TV when it was broadcast. There would be X episodes in a season, and they would air whenever they were scheduled to air. And if you were watching a television at that exact time, you would get to see that show. And if you weren't, you wouldn't. And if you missed the episode, you wouldn't get to see it. Or at least, not for months - until you hit re-runs (remember re-runs?). And because there were only 3-5 broadcast slots at any given time period (the three networks, PBS, maybe one local station), those shows were designed to capture as massive an audience as possible.

What that does to storytelling is obvious. Television had to be almost entirely episodic - you had to assume that your audience would likely miss some of the prior episodes, and so each episode has to be intelligible without prior viewing (other than the "previously on" pre-roll). And it has to be as broad as possible, something that will appeal to many tens of millions of viewers. Which means the types of shows you saw in the 50's through early 80's. The show would have a genre, a premise, and a set of characters that would not change from episode to episode throughout the season, for the most part.

Westerns, sitcoms, variety shows, crime dramas - the sort of thing that is basically just rerunning from first positions every single week (except soaps, both daytime and primetime). Everything got wrapped up in an hour (or half-hour). Columbo or Kojak solve the case, things are back to normal on the Ponderosa, Gilligan and his friends are still on the Island, and all the guests leave the Pacific Princess or Mr. Roarke's Island. You don't really have character arcs, you don't really have characters evolving or growing, and it's hard to deal with any really chewy issues when you have to draw 20+ million viewers to stay on the air. Sure, there would be "very special episodes," and you might have some big plot points that would change the status quo (a character would leave or enter the show, there would be a marriage or a baby or whatever). But for the most part, you could pick a season of the show and watch all the episodes in any order you wanted, and it would pretty much

You mostly get low-quality crap. It's really hard to tell good stories with those kinds of limitations. Without good stories, you don't get good directors or writers or actors. If you wanted to tell a more complicated story, movies were the way to go.

It really wasn't until the VCR in the early 1980's, and Hill Street Blues appealing to the type of audience that could afford that very expensive tech in the early days, that you could have the sort of television story structure we have today.