I think it ultimately came down to Rondo ok'ing the trade. Not that he really had a choice but I don't think teams want to commit to the trade unless Rondo signs an extension or gives them a verbal agreement he would. As soon as Rondo said he wouldn't re-sign with the Kings, the Kings pulled out.
In a way, I think the Dallas trade was the best possible scenario for Danny after listening to different offers and Rondo was ok to re-sign with Dallas
This is all my speculation