If you'd like to post a moveset for an older generation, then you may do that in the existing pages for each Pokemon -- there are plenty of sets like this already. Likewise with sets for Pokemon in different metagames; if you have a set for a tier the Pokemon is not explicitly a part of but is still allowed in, there is no reason why you can't post it in the main thread.
The reason we don't allow separate threads for each and every variation of the game mode or individual circumstance, is that we don't want twenty questions on the site for each Pokemon. Pokemon in PU would theoretically have a thread for all six tiers they are allowed in, then another seven threads for each generation. If we start allowing that, then a number of problems would arise:
Moveset threads already fill and clog the main navigation of the site a ton. If we started allowing more, then it would get clogged even more.
If we started allowing threads for circumstances like this, then people will eventually just start asking questions that get more and more niche, thus more and more unhelpful. I don't want to use a site where stuff like 'What is a good Gen 3 OU moveset for a Lonely nature Effect Spore Breloom?' is allowed.
Having multiple questions would distribute responses for individual Pokemon across multiple threads. I don't want to end up looking through five different threads to find good sets for one Pokemon.
Eventually our existing threads, which have years' worth of content on them, would become obsolete because there'd be a more specific thread you should post your set on.
Allowing more than one thread would kill the idea that you can click on a moveset thread, and figure out what sets were good from that. If a lot of Pokemon are using a certain trait in a moveset thread, then you can assume that trait is good. But if we started splitting up the threads based on those same traits, then it would be impossible to tell which were good and which were not; there'd be nothing to compare to anymore.
Categorising sets in this way would become really messy because the current threads already have sets that were posted for past generations, specific tiers, etc. We'd have a problem where sets are posted in the wrong place, right from the getgo.
As you can probably tell, I've thought about this before. :P
