It is fair enough to counter that these were trained police officers who knew Floyd had rights, knew they were responsible to care for him while he was in their custody, and yet unreasonably (a) maintained him in a dangerous hold and (b) failed to provide first-aid when his breathing and pulse failed. So yes, they could be convicted in federal court. Still, the case is far from a slam-dunk. If the defendants were acquitted, that would enable them to claim vindication and undercut the accountability for police brutality that the Minnesota convictions had accomplished.
As a law-enforcement matter, the federal prosecution is not defensible. It is a needlessly redundant expenditure of federal resources to achieve a result that will already have been achieved by the state prosecutions. It puts defendants in jeopardy a second time for the same wrongful actions. Convictions would not advance accountability, but there is a significant risk of acquittals that would undermine accountability. And, under the guise of prosecuting an abuse of civil rights, the Justice Department and its Civil Rights Division are quite intentionally violating the civil rights of the defendants to fair criminal proceedings in the state court.
Of course, as a political matter, the prosecution makes all the sense in the world. The Biden administration needs to appease its angry base on the hard left.