I noticed that too! Just a shot in the dark, but I'm guessing that family members get more chance of getting elected if they stay inside cities, which is more likely to happen if you don't conquer anything.
There was a thread in the Ludus Magna a while back, I think, about how the AI expands-- something about the line-of-sight-- for example, the Seleucids almost never take Palmyra but aim for Dumatha even though Palmyra is much closer than Dumatha.
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