I think there is more reasons why Reddit is commonly credited with being the origin of memes over /b/. If you were to ask the common internet surfer (maybe even a 4chan user) what they think about 4chan, some will say that they've heard of it but never ventured there due to the negative connotations that come along with the site. 4chan is most infamous for being the starting point of anonymous and there are many people who are scared of that. With putting aside the self gratification that the point system on Reddit has on its user base 4chan was already at a disadvantage due to its social standing.
4chan suffered for its reputation, but not because it developed a stigma. After a year or two most of its new users came because of the stories they heard about /b/'s antics, and so tried to reenact them over and over. Even the ones that were actually funny the first time stoped being funny the billionth, but anything other than endless repition got drowned out.
Let's also not forget that SomethingAwful migrants were the origin of the SRS movement.