why did framers think it was neccessary to give national government power over states?

Answer 1

When you read the writings of Alexander Hamilton, you may be surprised to find that he wanted the states to have more powers (infinite) as opposed to the national government whose powers he wanted spelled out (finite). Of course, the national laws have precedent over state laws but the states, in theory, have more jurisdiction when it comes to making laws. In other words, more issues are suppose to fall to the states. A side note to this is the theory behind the House and the Senate. In the national legislature, the House represents the popular sentiment (the people) and the Senate, whose members were originally selected by the state legislatures, represented the interests of the individual states. It was very common before we went to the direct election of senators for states to pass resolutions directing their senators how to vote. The idea is that the national government will not over step its bounds because the upper chamber which is controlled by the states will be hesitant to take away powers reserved to the states. In other words, senators want to keep their jobs so they won't vote for anything that makes their constituency (the state legislatures) angry.

Answer 2

The framers of the constitution created legislative, executive and judicial branches of the government which were for the benefit of all people. It created areas of jurisdiction for services which were best served federally ie army, roads, postal service, immigration, tax and toll collections. States could set their own agendas so long as they did not conflict with or usurp federal obligations.