The team may have wanted to prevent a dog-pile on the SPQR. Vanilla R:TW is basically set up as Rome vs. the World; but Rome is a coalition of three strong factions, with a fourth holding Latium. In EB, Rome is far weaker.
That said, don't overrate the importance of this mechanism. It just means that the A.I. expands north if there are no pressing reasons to go elsewhere. A friendly relation with the northern neighbour will override this, as do ongoing wars, and the urge to attack the player's faction.
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