I agree. Probably game theory reasons. Don't steal so much it will cause the community to unite and hard fork. 30 million is enough to retire and probably not enough to cause hard fork and rollback of txs. They played it safe.
I think this is what happened and it makes sense if you think about it. If they steal enough to get rich but not enough to force a rollback, then they can keep their ill gotten gains.
30 million is enough for a small group of hackers to live comfortably for the rest of their lives.
