Most things already are. But there are many things that do need to be a federal concern, because a state does not exist in a vacuum, nor is it a (metaphorical) island. Things like the environment and pollution, for instance, need to have a federal component. One state may decide to relax environmental protections so that they can encourage factories and jobs. However, that pollution does not limit itself to the people that decided it. People outside that area, in other states that do have environmental regulations, will also be affected.