In general, lands in river valleys are good for agriculture of all sorts. Cotton became "king" in large part because that was something that was in great demand.
The Mississippi River valley is good for cotton because it had been an area that got flooded repeatedly over the years. Floods are good for agriculture because they bring rich topsoil and deposit it as the flood waters recede. Therefore the Mississippi River valley had very rich soil. The flooding also produces alluvial soil, one of the best types of soil for cotton growing because it offers holding capacity, drainage, and aeration. The area was made even more conducive to cotton production because of its climate. It has long, hot summers which are necessary for cotton which is native to warm climates.