I bet if you did full forensics on the ti frames that have broken you would find workmanship or design issues or simply too light a frame for the job in almost every case. Titanium, like steel, has a fatigue limit. Keep stresses below that limit and it will remain fatigue free for a very long time but get sorta close then add some welds that aren't executed very well, perhaps thinning the tubes adjacent to the weld ends in a place unseen and the stresses there now start creeping above the fatigue limit. Ride this bike long enough and the crack starts. Poor design of stressed areas may have the same outcome even if the workmanship is good.

The workmanship required to do ti right is not a trivial matter. By comparison, a lugged steel frame is incredibly forgiving.

Ben