Why Do Some Series Have Long Gaps Between Seasons?
There's a key difference between a long-running show like, say, One Piece, and a late-night show like Kuroko's Basketball (or, this season, My Hero Academia). The former is a very mainstream show aimed at kids, which airs on Sunday mornings. These are shown with the full participation of the TV network as a regular show, and have open-ended sponsorships that allow the show to go on and on for as long as it can. These are an endangered species these days: nearly all series now run as "late night" anime, which work entirely differently behind the scenes. They're aired as informercials for for DVDs and Blu-rays of the show itself, its original manga, or other related things. Originally these late-night shows were aimed just at hardcore otaku, but once the business model became established, it started being used for every kind of anime.
The economics of a "daytime" kids' show and a late-night show are very very different. Kids' and family properties are designed to run as long as possible, since their primary reason to exist is to sell merchandise. The merchandise makers sponsor the show through the TV network, the TV network pays the producers, the producers pay the animation studio. Everybody's happy, and as long as the sponsorship money holds (i.e. the toys and stuff keep selling), the show continues.
This system produced a lot more bombs than it did hits, and the thing is, anime has such a long lead time that putting the breaks on a show that bombs can take literally months. By the time the sponsor and/or TV network decides to pull the plug, there might be another dozen episodes in various stages of production, and all of that still has to be paid for. It's a huge waste of money. So now this system is reserved only for the "surefire hits", i.e. proven kids' and family properties like One Piece.
Everything else goes in late-night infomercial timeslots, which are purchased outright by the production committees, who find sponsors on their own, and plug their own commercials into the programs. They have their own plans for making money (usually involving DVDs and merchandise), and the TV airing just acts as a commercial for those products. And to mitigate risk, they only produce them a season at a time. That way if it bombs, they're only out for those handful of episodes they set out to make. And if it's a hit, like My Hero Academia, they'll start another season going.
But the engines take a while to get moving again. The team has to be reassembled, new scripts have to be made, and there's so much to do before the show can see air that it's usually only possible to make a season of anime once a year at this rate. Even more importantly, there is so much anime being produced currently that all of the studios are currently booked several years in advance, so just finding a slot on their schedules to get it made can take a ridiculously long amount of time. Occasionally a production committee will get daring and green-light two seasons at a time, but that's a rarity. It's all in the name of not taking unnecessary and expensive risks.
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