Suppose someone uttered a forged document across state lines with fraudulent intent. This looks like (attempted) fraud and (completed) forgery in both states, as well as (attempted) mail/wire fraud federally. However, 18 USC Ch 25 clearly applies only when the intended victim or misrepresented party is the United States/agents/etc itself.
Since attempted fraud is basically a predicate offense of forgery, presumably the latter is more serious. Given that federal statute treats "interstate commerce" very liberally (the reach of mail and wire fraud is extremely broad), why didn't congress create a separate crime for interstate forgery? (Start with a copy of 18 USC Ch 25 with all references to "the United States/etc" replaced with "any person, across state lines" and adjust from there.)