Saverin eventually settled out of the court, for 5% equity. But why didn't he proceed for his original 30% equity when it was clear that facebooks majority shareholders failed to exercise the fiduciary duty?
Because he wanted to
Why he wanted to only he and whoever he's told knows.
An out of court settlement is simply a deal that comes at the end of a negotiation - possibly a long and arduous negotiation. People make deals based on what they think is in their own best interests. This may not be going to court for many years, funding that litigation for an uncertain outcome that might be better than what's on the table right now. Or it might be taking the money and getting on with your life.
They did not, they were sued and settled. Zuckerberg simply managed to dilute the shares as CEO when saverin was not present. Also, Zuckerberg was listed as an officer of the LLC and could make these changes before saverin found out.
A bigger problem is how he got away with stealing the entire company from Paul Ceglia, who presented emails that Harvard never challenged showing that he owns 84% of Facebook. It is likely that once Facebook is less valuable and has a smaller legal budget this case will go to court and be decided in his favor.