But to me the main reason not to is there is nothing to gain from having a separate site. Tags work for stackoverflow.com if you only want to see lua-related questions, and if there were separate sites for different technologies, it would be unclear where to post a question that involved more than one (like pretty much all of the "lua binding to XXX" discussions here - do they go in the lua stackexchange or the XXX one? Better not to have to decide.
