Why did farming replace hunters and gatherers?

Because hunting and gathering takes way too much work and everybody in the clan or group has to participate. With farming, only a few people have to do it, so it allows for other people to do more skilled labor such as blacksmithing or making clothes, etc. It also creates a more stable and reliable food supply. The whole clan doesn't have to move around once the food is gone like they do in hunting and gathering, and they can have more kids because it's easier to support them in an agricultural society.