The primary reason would be that the mail list has been around a lot longer than this forum has.
Also, I think having to subscribe eliminates many of the way-off-topic posts that you have to wade through on the TMG forum on this board. It's usually not worth the effort.
And then there is the active/passive difference between a board and a mail list. With the mail list, the messages get sent to the participants - they don't have to go looking for them. That means that more people will read them than would if it were posted on a board where people would have to go looking for them.