There is a lot of argument about why most people are naturally right-handed. There is no doubt that this is the case; you find a majority of right-handers all over the world. A common argument is to do with brain function, especially language; nearly all right-handed people process speech on their left side, while only about half of left-handers do so. But this doesn't explain everything.

It's also thought that being right-handed might give a slight advantage, as certain illnesses, especially mental illnesses, have been found to be more common in left-handers. There is also some evidence that left-handers are more aggressive and given to fighting, which may or may not be an advantage. They are commonly thought to be more successful in sport and combat, but this is may be because right-handers are less used to dealing with them.

You can learn more about the latest theories on handedness here.