Why did cotton become the leading crop of the south?

There were various reasons for that but they require some historical context. There were quite a few big industries in the US before cotton, like tobacco, sugar, rice and others. For example, tobacco market was unstable and the tobacco itself was hard to cultivate and it wore out the ground.  On the other hand, cotton is a sturdy plant able to survive in plethora of climates. Not to mention it was much easier on the soil than the tobacco was and there was no competition for it in the Caribbean like there was for sugar. Also the rule of supply and demand applies here because cotton was requested by the booming textile industry.