Why do most anime have only 26 episodes?

Japan has 4 broadcast seasons, roughly split up into summer, fall, winter, and spring. At the beginning of each season, dozens of old series end and dozens of new series begin, all within a few weeks of each other. There are approximately 13 weeks in each broadcast season. This is why so many anime are around 13 or 26 episodes. A 13 episode series ran through one full broadcast season, and then ended. A 26 episode series ran through two seasons, and ended. Of course, you will often get an anime that is 11 or 12 episodes instead of 13, or 24 or 25 instead of 26, because they ended a bit early in the season or started a bit late in the season, but the the AVERAGE of 13 and 26 is because of how the broadcast seasons are set up. The series that are 50-52 episodes are the ones that run for a full year (4 broadcast seasons), 52 weeks.

It all depends on funding...Most of the very niche anime are about 12-13 episodes (if that...I mean, there are 1 episodes OVAs). Very popular anime, like Naruto and Bleach, have a lot of funding. That is why they tend to reach of 100 episodes. It also has to do with when a production is aired and on which network.

Promotions + famous studio + large fanbase/demographic = lots of money to continue the series

Anime are licensed by TV stations for set number of episodes. For example, if there is a contract for 26 episodes, then 26 will be made. This is why many anime that have manga that continue on when they don't may be altered or shortened (e.g. Elfen Lied) to fit within the amount of episodes they've been licensed for. Also, many anime are released seasonally, so 26 episodes means 26 weeks, or half of a year. Most anime will run for a season (12-13 episodes), half a year (24-26 episodes), or one year (50-52) episodes. Of course, we've seen many anime that run into hundreds of episodes, like Pokémon, Naruto, Katekyoushi Hitman Reborn!, or Bleach. These anime probably have special contracts that simply require their animation companies to animate the story.

Sometimes you need to analyze a story to guess how many episodes a new anime will have. If the story is simplistic and there are a relatively small number of characters, it will probably be short. Longer anime usually have an extensive cast and a complex storyline. Those things may not be true for all anime of those respective natures, but they seem to work most of the time.

I hope that helped.

I think it depends on the studio. Anime episodes range from 13 to 26 episodes even shorter and longer. It's hard to keep up with the manga as well. Take a look Bleach and Naruto. They are long running anime and mangas but the studio has to create filler episodes (stories arc different from the manga) in order to keep up with the manga.

Most Anime Episodes

it depends on how the show goes

Because that's the way the it is.

Because the people who created that show can't think of anything else after that, or they get bored with the show they created and make another one.