Star Trek: Discovery has around 15 episodes in each season, and Picard is going to have only 10. But The Next Generation and Voyager had nearly 30 episodes in each season. Why don't they produce longer seasons anymore?
It is starting to be the norm for shows that air on subscription channels. Most new shows on these “on demand” channels only have 10-12 episodes.
The most common reason I’ve read is that they don’t have to run a factory churning out 22-24 episodes so they make a profit from the commercials, since you’re paying them directly via the subscription fee.
The second most common reason I’ve seen is that it allows them to concentrate on quality instead of quantity. The fewer episodes they make, the more time to focus on writing, and the more money for production value. The fewer special effects you have to do, the more money you can throw at them. Honestly, it’s not THAT bad. When there were 22-24 episodes per season, at least 7-8 of them were basically fillers to meet the contracted number of 22-24.
Lastly, the phrase “always leave your audience wanting more” comes to mind.
Nobody has pointed out syndication.
In the non-service based model (ie - pre CBS All-Access), there is/was a long term financial benefit to produce as many episodes as possible, so you can then sell them to third-party networks to re-air.
This even affected Star Trek The Original Series:
At that time, if a series managed to hold on for five years, building up a package of 130 episodes, it was an easy sell to syndication where the big money came from
source: These Are the Voyages: Season One - Marc Cushman
"Back then, the thinking was you needed 150 episodes or so to have a strong syndication package", said John D.F. Black, Star Trek's first season associate producer. 'We made 26 to 30 episodes in a season in those days. So the plan was to keep Kirk and his crew out there for five years. That's why it was a five year mission. And that's the only reason."
source: These Are the Voyages, Season: 2 - Marc Cushman
In the old days of television, you needed around 24 episodes per season to have one per week for the Fall to Spring run, with time off for Summer. You were filling a literal spot in the weekly lineup, so you had to produce that many.
Now it is a lot more fashionable to have half (or less) seasons, particularly when you aren’t trying to fill a time slot, because all of your content is digital on streaming services. It is also less expensive and therefore less risky to produce fewer episodes.
As has been pointed out, while modern episodes are more expense, even 4 times more costly, it is more easily affordable, because you don't need to budget for as many episodes.
This is almost off topic, because it is a change in how TV is made, not just the way that Science Fiction and Fantasy is made.
It was also revealed that an average episode of the first season had ultimately cost US$8–8.5 million each, making it one of the most expensive television series ever created.
Although the final season of GoT was even more expensive.
Each episode of the show's eighth and final season, which debuted on Sunday, cost $15 million, according to Variety, due to its film-like production schedule. The final four episodes are 80 minutes long each, and one battle reportedly took 55 days to shoot.
This goes directly into the second major production reason, Time. The more money you're spending, the more time you're spending shooting and in post production, the less content you've got to screen for the same time investment.
The two articles are fairly intersting and have a lot more to say. Some of it about the way people watch TV, some of it about how actors can influence the schedules. Suffice to say, it's for a lot of reasons, but most of it comes down to TV being very different from 20 years ago. A lot of the time you're now really watching a 10 hour movie, not 20 different stories per season.
You're also forgetting something key: distribution changed.
When TOS came out, it was being run exclusively on NBC (when 3 networks dominated everything in the US market). Many network schedules of the day wanted over 20 episodes. Consider Gunsmoke (1955-1974). In 1966 (when TOS debuted) they made 32 episodes, down from 39. By 1974, they made 24 episodes. This was to facilitate the TV network seasons (airing one episode a week).
TNG (which was syndicated instead of being exclusive to a specific network) made 26 episodes per season (Season 2 was only 22 due to a writer's strike)
Modern TV isn't as seasonal anymore. Indeed, many series are released in shorter seasons due to streaming. Discovery and Picard are exclusive to CBS All-Access (or Netflix outside the US) and can be binge watched once the "season" run is done. Streaming hasn't killed the weekly release either (allows for social media to dissect and speculate from week to week and keeps subscribers paying). But with binge watching comes a disincentive to produce large seasons. Attack on Titan (anime TV series) had a large Season 1, but much shorter subsequent seasons, partially due to binge watching
Romain cited issues related to staffing and overproduction in the Japanese animation market. He went on to imply that anime production houses, in general, don’t have the up-front money to support the industry trend of delivering episodes in bulk — a result of the proliferation of binge-watch streaming.
There are several answers focusing on the show and broadcast formats and the financial constraints. Let's take another look from the story telling perspective:
It seems the examples you named are shows that are not "episodes" but a story split into pieces.
In many old series the pilot would set up a general setting, then each episode would play out a little concise story in this setting without changing the overall setting (much). At the moment however, over-arcing story driven series are more en vogue than such episodic shows. These shows typically focus on an overall story line that develops through the individual episodes and has a fixed ending, the background setting is prone to change in a much stronger form each episode. Such stories are often more fitting for shorter runs, as you need to make sure the audience can follow and the story does not feel too drawn out.
The difference between network television and streaming sites. They have different goals and as such may produce different strategies. Both want to maximize their profit but in network television that's on a per episode basis whereas in streaming it means more to get someone in to pay the $7-10 a month. The number of episodes is dependent on the number of subscribers it generates. You could literally break it down to a mathematical formula.
CBS owned the television rights to Star Trek but not the movie rights. With the success of the JJ Abrams movies, it is certainly plausible that CBS went a different direction with Discovery and focused more on special effects than story and characters which led to much more expensive episodes. They tried to emulate the movies in an attempt to build their new streaming service to the level of a Hulu or Netflix.