So this is something my friend and I have always wondered about. We didn't play when he came out so if this was an issue at the time or already brought up I apologize:

Why does Justinian have the trait that makes him +2 levels while in a discard pile?

There are the obvious reasons, like he is harder to Raise him from the dead and cards that may remove him from your discard pile that depend on lower levels and whatnot, but we are curious about why design singled him out? Was there some specific combo they did not want him to work with, or some storyline reason?

Inquiring minds want to know!

captain karkos Tzin in design/playtest played with the graveyard and the levels of guys there and Justinian was originally designed to interact with that, or so the story goes.

But then they changed Captain Karkos Tzin and left Justinian alone? Weird.

there are other cards that look at level in discard piles, it just made justinian harder to remove/revive.

Discard pile mechanics were a strong theme of Death's Bargain. Go through the card spoiler and you'll see all kinds of interaction in the set.