It’s because they walked away from the tried and true broadcast television model:

long seasons, often 20+ episodes

There's far more competition in television now than in the era of regular 20+ episodes a season shows. There's simply more TV shows, and thus more options for audiences. Actors in TV now are also regularly in movies and many other shows. Many actors now worth hiring simply do not want to tie themselves down to 20 episodes a season, and define themselves by one show. Audiences also expect higher CGI quality than you would usually get in 20 episodes a season shows.

There's a reason it's primarily only network procedural shows that stick to that formula.

This model was created out of necessity: the technology was broadcast television, no video recording technology cheap enough to distribute episodes in bulk. However the model had a number of advantages. First of all, it created audience loyalty because shows had a consistent and timely presence in people’s lives. People would gather around the water cooler or on the playground to talk about last night’s episode, since it came out for everyone at the same time.

I mean, the watercooler culture is also gone primarily because of the increase competition. Two avid TV watchers now could be watching an entirely different array of TV shows with zero cross-over because of the prominence and accessibility of options to audiences - especially from the Non-US world. That was simply far less likely to happen 10 years ago, Shows like Lost in the 00s were not indirectly competing with Korean or German shows. They just weren't.

It also gave shows evolving cultural relevance, as longer runs gave the writers opportunities to respond to current events, fashion trends, or changes in technology.

Shows that try to incorporate contemporary trends within their plot tend to not age very well.

Lastly, it was immune to the “binge burnout” and “cramming amnesia” effects, two terms I just made up (but describe real phenomena). Everyone who has binge watched a season of a show knows the bad feeling you get at the end. It’s a mixture of burnout but also of grief, because you’re forced out of a world you’d immersed yourself in. Cramming amnesia should be instantly familiar to anyone who has crammed for an exam, and everyone knows that spaced repetition over the long term is much better at building memory, whereas cramming leads to forgetting.

Not all networks use the binge formula: Apple and Amazon don't always do this. (Apple never does this).