To everyone here just to argue that rdf was not part of wrath, a couple simple google searches can show you that you are wrong.
Wrath release date - November 13, 2008
Wrath patch 3.3.0 - December 8, 2009 (this patch added Cross-Server Dungeon Finder)
Cata release date - December 7, 2010
So RDF was part of wrath for almost exactly half of the expansion. And as stated by blizzard themselves when talking about the success of wow, the population had substantial growth mid way through the expansion. So more people who played wrath were exposed to the original RDF than people who were not.