I'm in the middle of a rewatch(I've lost count on how many) and I just finished Out of Mind. I thought of a theory as to why Delta was the closest to becoming "human" out of all the Freelancer AI. Since Delta was a break off of Alpha's logic, he has a drive to constantly understand events and people around him. Having this sense of curiosity is(in my opinion) one of the core tenants of what it means to be human.

We, as a species, throughout all of history, constantly attempt to understand one another. We developed the spoken word so we could communicate with one another. We developed the written word to share ideas with those that come after us. This drive to understand each other is just part of what makes us human. Delta constantly tries to understand why York and his friends do the things they do. From a purely logical perspective, he can't understand why someone would risk their life for the sake of someone else. Yet, he WANTS to understand. And that is why he stayed with York in his last moments. He knew, or at least was led to believe by PFL that he would be deleted should he stay behind.

This is just a theory and obviously is just my own head canon, but I figured I would share it.