Cause there isn't any more seeders (ie. those that have 100% of the files).
When a older series, like this one, is broadcast only once a week. It is very easy for people to give up on seeding after a month or so.
Btw, if a file is around 9x% completed, you may still be able to play it. I use MPC-BE, available from https://sourceforge.net/projects/mpcbe/