The Bulls would be less willing to deal Butler within the conference. That's always a factor.
We don't really have a boatload of disappointing young former lottery picks to trade like the T-Wolves do, besides Smart. Our Dunn would have been Brown. That would mean we traded two top three picks for Butler, which most people would consider more than fair.
If it was only the third pick for Butler, I think the Celtics really think they can get Hayward or Griffin this offseason, and may have thought those players were better fits and didn't want to lose the assets. Or, they think they can get George for less?
We also don't have many bad contracts on the team, making deals hard because we have no "filler."
The 3rd pick in this draft was worth a lot more than the 7th.
All this being said, I think we do need to make some kind of trade. There's just not enough minutes to go around, and potentially we may lose Bradley for nothing. They do need to clear at least some money to make a max offer, so we know that has to happen somehow.