The stories are held in fields on a database, same as everything else here and each field has to have a finite size which in this case is set to 10k characters.
A good guess, since this is normally the case for setting text limits.
In this situation however it's more to try and help authors give the stories a book-like feel, with chapters of roughly equal length. 10,000 characters seemed about right for most people.
There's also a minimum limit of 2,000 characters to try and stop people using the stories section for posting comments etc.
