In the case of the Obama era Pot rules, the federal government had basically said "Rather than enforce our unilateral law, we're going to allow individual states to decide how they feel and only enforce the prohibition in states that continue to request it." Which makes perfect sense. Once enough states had legalized it, it would create momentum to legalize it on the federal level. It made perfect sense for the federal government to get out of the way and let that process play out.
Enforcing the federal law in states that legalize it is just going to set up a battle between those states and the federal government, with a lot of very costly court cases and headaches for the various agencies tasked with enforcement.