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.

> But there are many things that do need to be a federal concern

I completely agree! Most of them are already explicitly listed as federal concerns, in the Constitution; we should amend it to include environmental concerns for the exact reason you note.

But I see no reason why e.g. education should be a federal concern. Let the states do as they will.

Because there need to be national standards for education. The citizens of a state do not exist in a vacuum, and being taught Creationism instead of actual science is going to do great harm to those kids ability to get jobs, which is going to have an impact on many sectors of the economy.

> Really, in a federal republic aren't there quite a lot of things which can safely be left to the several states?

Well, you hope so, otherwise a federal republic is a good idea. OTOH, simply because you have a federal government (or even that it was a good idea a century or two ago) doesn't mean that a federal republic is a good idea, and even if a federal republic is a good idea and there are quite a lot of things that make most sense at the state level, well, that doesn't mean that every particular thing belongs there.