This is because a mailing list, in fact, is representing group and the subscribers do have a strong sense of community. The message a user send will go directly to each mail-box of all users. On the other hand, a message on the mailing list, in a sense, is much more intrusive. It will, however, be naive to assume that every message appear on the mailing list is getting an equal attention. I, for example, do read BurmaNet everyday.
Some messages are to be read seriously.
The others are left just to check heading or skimmed through the contents. Not all the users on the list are likely to be interested in every conversation made by a few persons on the list. This is most common by the standard news senders. The users do appreciate to be able to read the news readily available from the mailing list. The same news item being posted simultaneously by different users may not necessarily be a bad thing.