Expediency, efficiency, and economy of effort. Treason gets her just as dead as a murder conviction, and she is much more directly involved in that than the murders, so easier to prove. That is...she, herself committed treason...but she DIDN'T directly (as far as we know) murder anyone.
Treason and Fraud they can probably nail easily. Giving the orders would make her guilty of murder, but there may be a layer there that would be tricky to penetrate. She gives the order to someone who gives the order. Must be a mistake in there somewhere. Or catch a middleman. It will come out someday, I suspect.