That's because they are the ones actually studied.  I imagine it's more like 80% of the general population as well.  Just about everybody harbors it, which doesn't mean they all need treatment however.  Don't believe me...go get a nasal swab... I lay odds you are positive. 

MRSA is just the now scientifically accepted name for good old fashioned Staph aureus, which most of us have...somewhere.  The reason it's Methicillin resistant is due to overuse by the medical profession, particularly in hospital settings, so that form of it is more common in hospitalized patients and health care providers.   Bacteria mutate very quickly, and their new genetic mutations become resistant to formerly useful antibiotics.