The honest answer is that nobody knows for sure. There is some evidence that, overall, being right-handed may give an advantage - left-handedness can be associated with difficulties in learning to speak and even with some types of mental illness - but the evidence for all this is not very strong (as a left-hander myself, I've certainly never had any trouble speaking!). On the other hand, there is also evidence that left-handers may be better at some sports and physical activities, which could be why they have tended to survive - handedness is about 25% inherited, so if being left-handed was just a handicap there would probably be few, if any, left-handers around.
Basically, though, the question of why some people's genes cause them to be left-handed and others not is still waiting to be answered. The most likely answers are either that it is down to random genetic variations, or that, as left-handers have usually been expected to hide their handedness and 'conform' to the majority, there may be many more left-handed people than we know.