Basically, cable TV and DVD box sets allowed for more complex long-form narratives. On-demand streaming has solidified that.
In the old days you had a handful of broadcast channels and you had to catch your favorite show at the time it was broadcast or remember to set the VCR if you knew you'd miss the broadcast time. If you missed an episode there was no way to catch up unless the show was re-run probably years later.
Narratives were kept pretty simple and many shows were purely episodic. Those that weren't led with a "previously on..." segment. Even groundbreaking broadcast shows like Hill Street Blues were largely episodic. It wasn't in the interests of TV producers to make something that people would need to see every episode of. That wasn't a realistic expectation for most viewers. Viewing figures would plummet as people missed episodes, lost the thread and tuned out.
TV was also regarded as a lesser medium to film. If an actor who had worked in film appeared on a TV show you'd know their career had shifted down a gear and they probably needed the work. Those actors who graduated from TV to film (Bruce Willis for example) didn't look back.
Things started to change with widespread cable adoption. Although there had been some shows in the 80s (like Hill Street Blues that I mentioned previously) that pioneered quality storytelling on broadcast TV they now had an environment much more suited to them. Suddenly you effectively had instant re-runs. Episodes would air several times in a week. +1 channels allowed you to still see a show if the phone rang at an inopportune moment. Season marathons became a thing.
That environment allowed for narratives that required a much higher level of viewer engagement. It was easier to stay caught up and the long-term rewards of a more interesting story could lead to a fanbase that would watch every episode, sometimes more than once, for years.
DVD box sets became huge in the 2000s. Many shows would immediately publish a box set at the end of a season. They sold really well and shows like The Wire became huge hits that way. As word spread about an unmissable show, people who had missed entire seasons could go back and watch from the start.
Before long a move from acting in movies to acting in TV was seen less as a step down than a second act in a star's career. Actors started to move more freely between TV and movie work as neither was seen as inherently more prestigious.
Now we have streaming services and a generation of kids who have never known broadcast schedules. Many services still ration out shows one a week through a season but Netflix drops entire "seasons" at once and promotes the idea of binge-watching. Seasons can be designed like 8-12 hour movies with each episode flowing into the next, complex interwoven plots, multi-season arcs, etc.
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