What are the important events in my life? Well, as time goes on, the significance of any given event decreases. Hence, for any past time period, the density of "significant events" over that period will decrease as time marches away from that period. It's not that the events have changed, it's that our judgment is myopic.
Look at the second entry: "Eukaryotic cells, multicellular organisms". Is that an "event"? There are a huge number of absolutely critical developments lumped in there, and who knows when they occurred? When and how did mitochondria develop? How did the Krebs cycle arise? When and how did ribosomes develop? When and how did chloroplasts develop? (I could go on for a long while.) All of these were critical events in the history and expansion of life on Earth, but from the perspective of a billion years or more, it all gets lumped into a single development, "the cell".
The fact that those things fall on a straight line (just below 1 year per year) reflects human psychology, and not anything intrinsic about the nature of reality. We ask ourselves, "what was the most important event in the last two billion years", and then "what was the most important event in the last billion years", and then "what was the most important event in the last half-billion years", etc., up to "what was the most important event this week" (my wife made a really good meatloaf), and those events qualify for inclusion.
That said, I do believe that the pace of technological development is increasing exponentially. Unlike that graph, Moore's Law really is an objective measure.
The chart has no objective meaning.