Primaries vary state by state, in some you have a jungle primary where you don't have to register for that party. And railroad commissioner is a bigger deal than it sounds because it has a lot of power over semi related things.
That was only specifically because there was a special prosecutor. Appointing one is a way of showing that you're not stalling the process but it can lead to other less related charges.
Perun has pretty good videos explaining why, and a big reason is that Ukraine had a very good amount of AA as a former Soviet state and it was enough to keep planes out and they didn't want to send conscripts for political reasons so they sent armored vehicles without a full infantry complement which led to them being easy to pick off with the flood of atgms NATO sent.
Primaries vary state by state, in some you have a jungle primary where you don't have to register for that party. And railroad commissioner is a bigger deal than it sounds because it has a lot of power over semi related things.