There have actually been a few clips where they used a different rant from Downfall. If we continue discussing this, it will eventually devolve into an argument between those who think that this meme has produced massive sales of a previously obscure art house film and copyright absolutists who don’t believe this sort of use is covered by the Fair Use exception.
I think you’ve overlooked the significance of the Oscar-nominated cinema movie Der Untergang (Downfall) - one of the first major portrayals of Hitler by a German filmmaker, and thus a cultural watershed.
(IMO, it’s the astonishing quality of Downfall’s acting that makes the subtitled clips even more amusing, mind you. My favourite is the “Grammar… what’s the word…” one.)