Oh. Okay. That's confusing.

Why aren't DAs part of the Judical branch? It seems like they are a key part of our justice system.

Because the US system is adversarial. DAs work with the police to try and convict people. DAs are often very politically motivated in their decisions of who to prosecute and for what charges (throwing the book at people).

Judges are independent from that. They’re supposed to act as a check against the excesses of the DA office.