Just curious.
Well, speaking for myself, a lot of the time it’s because the thread contains old posters who for whatever reason, no longer post here, and it’s distressing to see them again in such a context.
Another reason is because all too often, the thread is bumped because of a “me too” posted by some latecomer chiming in, when the discussion was effectively over “back in the day”.
And sometimes the discussion was so ugly that we would all really rather not see it on the forum main page again, and certainly not because some latecomer needs to chime in with a “me too”.
Sure. Bring up “mad hatter” again. Thanks a bunch. 
If I may be permitted to hit the playback button yet again, on the theory that things said well once are worth referencing instead of repeating:
http://boards.straightdope.com/sdmb/showpost.php?p=4734799&postcount=7
Makes sense. Thanks.
I also wonder. They have never seemed to be a problem in the Cecil’s column forum. He’ll rerun a column, and rather than start over, you can pop up a thread from years ago. No harm done. If the original posters are no longer watching that thread, so what. Most people post and don’t really watch. And if for some reason they had it on email flagging (and still had that same email) then they might actually be interested in your new insight.
The Moderators as a group have agreed that pulling up ancient threads is, on the whole, not a good practice, for all the reasons outlined above and in the linked thread. Namely, the earlier posters may no longer be around, the discussion has been forgotten, the issue is no longer really relevant, etc.
There’s a different degree of discomfort, however, depending on the forum and the topic. In MPSIMS, for instance, a heated discussion about whether Fred should sue the son-of-a-bitch might be painful to resurrect six months later.
OTOH, Comments on Cecil’s Columns or Staff Reports are usually fact-based and more-or-less timeless. If Cecil says, “X is so” and Fred writes that there’s an exception, “X isn’t so on February 29”. Six months later, another poster says, “Yes, X is too true on February 29”. It’s not a big deal to revive the old thread: in fact, it’s probably better. Even if Fred is no longer around.
Thus, the practice is tolerated more in CoCC and CoSR than it would be in the other forums, and tolerated less in IMHO, MPSIMS, and the Pit.
Don’t forget Star Trek: Enterprise!
Many times, because it’s on UPN (which has odd and spotty broadcasting in the USA), certain fans will finally see the ep being discussed during the rerun period and wish to post in the ep’s thread. Us Trek Dopers see it as a part of the wondeful world of Trekking through the series.
Yes, we do actually discuss the show. More than 70% or so of the time. Except for Vivalostwages. But we let her stay anyways.
now reviving STar Trek and related threads may be good for those like me who have Never Ever seen them.
Ever. 