Mailing lists can get out of hand quickly. If you belong to a very active list, or to many lists, you could easily pile up several hundred messages in a day. This becomes a lot of information to sift through.
Mailing lists are very easy to create, but they are also easy to destroy. That really cool mailing list you saw last week may not exist this week. There is absolutely no central control of mailing lists, so they come and go with amazing frequency.
It is impossible to know which mailing lists are available. There is no organizational scheme or structure to them. It can be difficult finding exactly the list you want.