It's because of jurisdiction.
Normally, only states have jurisdiction over murders. The feds can charge murder if and only if the murder is connected to some other federal "crime of violence" (e.g. killing a federal official, murder-for-hire across state lines).
Here, the federal 'hook' was interstate stalking. But the federal stalking crime apparently isn't a "crime of violence" because you can stalk without intentional force.
Because the stalking charges don't legally qualify as "crimes of violence", the federal government doesn't have jurisdiction over the alleged murder.
What do you mean here? They usually get prosecuted by the state.
Because NY state law doesn't allow for the death penalty, and the federal government wanted the death penalty, they intentionally moved it to a federal jurisdiction.
There's a Twitter thread[0] that seems like a reasonable explanation.
[0] https://x.com/adamscochran/status/2017284770158485767 / https://xcancel.com/adamscochran/status/2017284770158485767
I know. But clearly they can be federally prosecuted and nearly all US crimes take place in an actual state (as opposed to other territories). Just saying that a state can do it is not sufficient to dismiss federal authority over the case. Since the guy conspired to kill a high-profile executive in another state, there seems to be federal standing for that reason (in my non-lawyer opinion). Even if he didn't cross state lines or assassinate such an individual, I think there are many ways the feds can take interest.
The same jurisdictional thing happens with state and local authorities as well, I think. You could be breaking local, state, and federal law at the same time and have 3 levels of authorities and many agencies that could all take you in, resulting in different prosecution. At least that is what I've been led to believe.