The best simple answer is "bad luck." A more complete explanation is that why an individual develops MS is due to a combination of factors which elevated risk. In the US, about 1 in 250 people will develop MS. Most of the risk of getting MS has to do with ancestry. Having a close relative with MS elevates the risk 50 times. However, only 2-5% of children of affected parents will be diagnosed with MS. People of Northern European descent, and those who grow up farther from the equator, are more at risk. However, any person can get MS. The risk to African Americans is about ½ of that of Caucasians in the US. Despite this, most people with MS do not have an affected family member. Some families break these rules and have many affected members, but the usual situation is that MS is less of a genetic illness than diabetes, heart disease, Parkinson’s disease, or Alzheimer’s disease.
MS in some respects appears to be a consequence of an infectious disease. Numerous investigations have failed to yield a consistent relationship with a single infectious agent. However, everyone with MS has a virus known as EB. EB is a very common virus which affects almost 90% of people, and infects their immune system in a lifelong fashion. It causes mononucleosis, also known as mono.
Most specialists who treat MS believe it occurs uncommonly at random following an infection with a common virus, leading to a disorder of the immune response which injuries the brain unnecessarily.