The federal government was established to do things the states could not do, or could not do well, individually. Denfense, international treaties, etc. The constitution was written to ensure that the federal government did not take over all power. If the framers had intended for the federal government to be responsible for the general welfare of each state's citizens, they would have made that clear. It says 'general welfare of the United States', not 'general welfare of the citizens of each state'. A trade agreement with a foreign country is an example of something done for the general welfare of the United States.
ra