I don't think anyone is claiming that he crossed state lines with intent to commit murder. And the crime of murder with which he was charged was entirely in the state of Wisconsin. Just because he fled back to Illinois after the commission of the crime does not make the murder charge a federal one.
EDIT: I mean, it would be very 2020 if his lawyers tried to claim that he did intend to commit murder and that it should be a federal charge, specifically because they got word that Trump would pardon him if that were the case.