I mean, why not simply say blacks have voting rights and cannot be enslaved, then let VA, SC, MA and NY enforce that however they wish (or not)? Other than the fact that in states where a majority belong to a national minority opposed to a law the law would be ignored, of course. Apart from that, I see little more efficiency in collecting taxes from 310,000,000 people and distributing them to state governments by population to then further distribute by need according to variable state laws rather than simply collecting the money at the federal level then dispersing according to need as determined by a single system. It only encourages states to prioritize a locally more numerous demographic over more numerous and needy national ones (or prioritize no one,) which would be fine if they were not doing it with federal taxes.

Personally, I think the most inefficient tax burden is at the local level for the above reasons, which are consequently the reasons complaints about taxes get louder in areas with higher local taxes and more local government programs: People paying more local taxes are getting less bang for their tax buck, not more.