Sales tax and income tax serve different purposes in most countries. And very little of that purpose is actually "raising" revenue - the federal government can in principle always print whatever money it needs and hand it out to states etc. But they do help ensure the fiat currency has irreplaceable value, that there's not excessive money in the system, and for income taxes also help smooth out inequalities. And it's often simpler or less fraught for state-level taxes to be used to supplement revenue raising rather than rely on federal government hand outs.