Here's the problem: the United States is a free trade zone. That means California cannot place tariffs on goods imported from Michigan or Alabama, and vice versa.
If environmental protection becomes a state domain, then companies that do pollute will all move to the least-stringent state, and still sell their products all over the US. A company in a stricter state will generally not be able to compete. Minimum wage also works in a similar way, as low-wage states take jobs from high-wage states. In other words, it is a race to the bottom.
The United States is also a free travel zone, so Arizona cannot deny entry to a New Yorker, and so on, which means the issue of universal health care also requires a strong central government. If one state subsidizes health care while another one doesn't, then sick people will go to the state with cheap health care, unfairly raising costs for everybody there. Just require the sick person to establish residency, I hear you suggest. The problem is that this shuts the door on people who move legitimately, but happen to be sick. In other words, each state must continue to provide whatever benefits it provides to citizens who depart for another state, at least for the duration of the residency period at the new state, or you'll have a (perhaps intolerable) gap in coverage. This is clearly a horribly messy situation that a nationwide coverage system can mitigate.
But you're free not to buy goods from polluting low-wage states, you say. The free market will take care of it, you say. How's that working for Chinese goods? Besides, the market is only a solution after the damage is done, and with things like carcinogens evident only after decades of exposure, punishing the polluters in the market is cold comfort. Worse, how would you like to travel halfway around the country to sue a polluter, while dealing with cancer?
Put simply, things that don't naturally obey borders require a central government. Pollution is not a state issue, because water flows from state to state, and the air is blown from state to state. Wages and other manufacturing costs are not a state issue, because we are all required to accept your products into our state. Communicable diseases. War. Long-Distance Transportation. Wild Habitat Conservation.
The US Federal Government may indeed be bloated, I'll freely grant. But unless you want to end free trade and movement among states, think twice about gutting the parts that require putting national interest above those of one or two states.