Handedness only has a heritability value of about 20%, which leaves the other 80% to be made up of potential factors such has individual variation (what keeps humans surviving, basically), and also overall body/brain asymmetry.
Everyone has a dominant overall side to their body, one that is better developed (this includes the brain). Nobody knows why the body develops like this however; an individual with a left-sided dominant body will most likely have a right-sided dominant brain hemisphere, and vice-versa.
But to be concise, nae c**t has a real clue. We only know genetics plays a small role.