One reason I can think of is that when it comes to United players, people are chomping at the bit to tear down our players for the slightest mistake and when I say people, its not just the ABUs but, our own supporters. For some reason the so called neutrals are not as quick to focus in on players at other big clubs in this way. They may get criticism but, no where near IMO what a United player gets.
If you think about what's going on with Maicon and Alves, they are probably making similar errors if not worse than Rafael but, theirs will hardly get any pundits lambasting them. So Rafael while at United has the burden of continuously having small mistakes being blown out of proportion and the myth of him being rash, hot headed and unreliable will perpetuate - which will only continue to remind the people in charge of the mistake in the Olympics despite being brilliant throughout till that point.
Funny thing is David Luiz who was responsible for Brazil failing on a lot bigger stage will not get the same treatment. Sucks for Rafael but, overall great for the club.