Everyone has staph on their skin but why would some peoples body's not fight it off like others?

Yes, we all have staphylococcus on the skin. Staph. is a microbe that is one of many called "normal flora". Normal flora on the skin, and other anatomical locations, compete for space with other microorganisms.

Different types of Staph. exist, some are more pathogenic than others. Depending on what type of Staph. it is and depending on the health of the individual, you may or may not get sick. If for example, one was immunocompromised for any reason, they might get sick quicker than someone who had a intact immune system. All sorts of diseases and conditions can lead to immunocompromise. Diabetes mellitus is a good example. Diabetic patients are immunocompromised because of the disease process. If you are under stress, physical or emotional, can have adverse effects on the immune system and lead to a compromise of the bodies defense system.