Why are most anime so short?
Why is it that most of the new anime that gets released are only 12 or 13 episodes long?
Because anime is an expensive investment and most producers want to see if a show will make a profit before they continue to invest
I thought the norm was either 12-13 or 24 episode seasons?
I think anywhere from 10-13 episodes is a perfect length for a series, but of course there are exceptions. Even in some 10-13 episode series, there are sometimes more than a few unnecessary episodes. That said, and as said above, the producers need to be sure a series will make money before producing more episodes.
The manga's sales also probably have something to do with it. A massively popular manga will be more likely to have a larger amount of episodes to start off with. After the anime premiere's it's all about sales and ratings pretty much.
I agree with what everyone has already said. Ill add that some series get episode counts based on how long the story will take. Some stories can be told in 12-13. Some in 24-26. And some in 50-52. This is only for a smaller percentage though where its episodes are predetermined by contract and not ratings. Most series are based on ratings and cost just like western television. Its all in the hands of the japanese audience.
Like most, when i first started watching anime i wanted every show i loved to keep going. But now as i matured I prefer shorter seasons. Less fluff. Most of the longer series i love( Naruto, Bleach, DBZ) all had a few sidestories that i didnt enjoy. Keeping it short and sweet and just hoping for extra seasons is better than a show degrading over time. There are still a few tthough that i hope get more and more seasons (DxD for example and Tokyo Ghoul).