Simply put, because the federal government’s power comes from how it allocates money, not from any document that actually gives it power.

The federal government is responsible for a) foreign relations (including national defense), b) handling interstate disputes (including interstate crimes) and c) implementing a national budget. There are a few other things it does, but those are the big three.

Of course, the last one allows it to say things like “pass this law to let us handle xyz or you get no money for it”, and most of the time, states will pass the law, as long as it’s for fundamental shit like roads, bridges, hospitals, food safety, etc. But for things like prosecuting crimes within a single state (which this is), the feds can’t do shit.