So as to let the states have their autonomy and not cause conflict between them.
Explanation:
USA was a federation of states and not just a single country. By limiting the powers of the Federal government the founders allowed the power to the states to have their own rules and thereby made sure that the country stays united.
More Federal power would have meant that some states would be unhappy and may cause tensions to the integrity of the country. The autonomy untied the states as they could manage their affairs while having the security of a large country.
