Federal prosecutors swear an oath to uphold all federal laws, and are generally bound by that oath unless an executive order by the president directs them not to enforce a particular law (in which case the president, not the prosecutor, would be subject to potential impeachment).
State and local prosecutors do not have the resources to pursue violations of federal law; they also do not have the jurisdiction to do so.
Federal prosecutors have several billions in resources to pursue violations of federal law, and are effectively not capped in their pursuit of "justice".
Doubly so given that multiple states currently have extremely uneven enforcement of federal law around the sale of marijuana.
Federal prosecution of drug laws regarding marijuana are fairly even. The difference is that states which have legalized marijuana make it far easier for federal prosecutors to find defendants than states where it is illegal (because defendants in those states do not distribute their marijuana in the public eye).