The tenth amendment to the constitution plainly states:
"The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people."
A lot of reading to catch up on what powers are delegated to the federal government, and plenty of interpretation has happened along the way.
Now, the legal question is why have Republicans ignored this order? It sets a real precedent, that's why. It nullifies States Rights. Republicans have long used states rights, sometimes successfully, sometimes not, to do things like nullify restrictions on gun ownership, immigration, and Devil's lettuce. States rights have been used to modify discrimination against the elderly and disabled, and limited the ability of rape victims to sue in federal courts.
States Rights is also used as a Dog Whistle for a number of other things, like desegregation, same sex marriage, and reproductive rights. A lot other this rhetoric was used by the Dixiecrats, who en masse moved to the Republican Party when the Democrats became too liberal on segregation.
So to the issue at hand, States Rights is an important tool in the Republican Party's toolbox. One they wish to not have nullified. And a lot of states rights ideology makes sense no matter what party you are in. After all, the tenth amendment was put in there for a reason.
And despite the years of obsequious Marching to Trump's demands, at least some Republicans understand that Trump won't be in office forever, and a Democrat will eventually land in the office, with all the powers Trump declared as his own.
Wikipedia has a nice writeup, https://en.wikipedia.org/wiki/...