Why most people are RIGHT -HANDED?

Many animals show a preference for one side of the body over another, but the split between right- and left-handed varies.

• Seven out of 10 chimpanzees are right-handed.
• Almost all kangaroos are left-handed. 
• In cats, males are nearly all left-handed and females are nearly all right-handed. 

Humans have a higher proportion of right-handers than any species, with left-handers making up just 10 per cent of the population.

The perfect reason is, "we are a tool-using species", and also highly social. We make use of things that are build by others. So, to match the hand pattern, people use same handed versions.
The very earliest flint tools, around two million years ago, don’t show a strong bias towards left- or right-handed versions.
 But it’s a big advantage if you can use the tools someone else has made, and, from about 1.5 million years ago, we seem to have standardised on the right handed versions. It’s not exactly clear why right-handedness won, but it may be that one side of our brain was already specialised for fine-motor control. One theory why left-handedness hasn’t been completely eliminated is that it provides an advantage in combat, precisely because it is rarer, and, therefore, unexpected. You can see this today in sports like tennis, where left-handed professionals are more common than in the general population.