Because most of the exclusive stuff was pretty rare already back then - it's like getting a cutting edge or running high M+ now (or at least I would say things like CM's were harder than current AoTC considering the skills of an average pug group). So imagine a situation where they re-introduce some of the old things that scream - 'I'm unique' and obviously most of the players would want to get it. If Blizz wanted to be worth its salt they would have to make it as hard as it used to be or otherwise what's the point. On the other hand, can you imagine the cryfest people would put out if they suddenly discover they are too shit (as the majority of the playerbase is too god damn awful for anything remotely challenging) to get anything? So Blizz would have to make stuff as easy as is currently queueable content or they could, indeed, make it actually difficult but then people would a) cry, as mentioned and b) just get boosted like no tomorrow and again - what's the point then? A lot of the old things are about the prestige - for instance, T3 has low-res texture and has recolours. The reason people pay crazy amounts for it is exactly that it is so rare (and I think they shouldn't have put it on the BMAH either). I very strongly doubt the vast majority of those who want long gone things want them because they are actually amazed at their appearance but rather because they are rare, unique or so on and for that reason they stand out.
And I'm not even discussing the fact that creating an authentic experience with all the changed stats, classes, spells, gear (no longer working tier-bonuses) and so on is pretty much impossible anyway.