The archive on NNTP is a first-class artifact that you can interact with in your newsreader via NNTP. You can do things like reply to old threads. The archive

The archive on NNTP is a first-class artifact that you can interact with in your newsreader via NNTP. You can do things like reply to old threads. The archive on a mailing list is a second-class artifact, typically hosted via HTTP, and usually people don't care about the quality of the archive's UX. The structural organization on NNTP is also a first-class artifact that everyone can refer to. A mailing list will go straight into everyone's inbox, with all their other mail, until they filter it away.