Tobacco, Rice, and Indigo were so work involved and Indentured Servants were expensive, so they decided to use African Slaves.
Labor mobility. In markets, supply is represented by companies while demand is represented by customers. Its not the demand of the product that pays, it is the skill set of the employees that determines the their degree of pay. If you ask yourself why a retail sales person is paid less than a doctor, you will obviously note that the difference in their pay is their skill. Businesses look for profits, and skilled workers drive profitability high hence they get to be paid more than low skilled workers.
The United States would be protected by Spain if a conflict with England arose.