"He could not force state and local governments to enforce federal law. That's due to the principles of federalism ..." DI #157
There are "principles" (your word) and there are laws. They are not always the same.
Years ago the standard was that the local police were not to "enforce federal law", but to hold suspects, persons without papers, until they could be interrogated / investigated by the proper federal agents.
But the feds were lackadaisical about holding up their end of the bargain thus snarling and squandering local police resources. Eventually, perhaps not out of spite but out of necessity the locals stopped holding the W.O.P.s
There's more than one way to skin a cat.
That might be part of it but it is not the real reason. They do not enforce it because doing so discourages their residents from working with the state or seeking state help. If you can't get residents to report crimes that they are victims of or may have witnessed then that is a big problem. Another one is that making it a priority will discourage those residents from medical services intended to protect public health. Also, many of them have citizen children who might then be kept away from those things that are intended to help them be functioning and productive members of society. It really makes no sense for them to enforce it. Yes the lack of federal action probably exacerbates that as the only real value the state/local governments gain from enforcement is removal of REAL criminals (as opposed to simple civil violators).