The other point to note about the estimates in the graph are that they have declined over the course of several decades, and noticeably so in the wake of the global financial crisis (GFC). This decline is common globally (Graph 3), which is to be expected as the neutral rate largely rests on global factors.
The causes of this common downward trend are less clear-cut, however. Many possible explanations have been proposed in the literature. Some are more plausible than others, and of course there could be more than one thing going on.