The chart clearly shows that there is a lot of proliferation, which is confusing to a lot of new users. Imagine what would be possible if all these fine developers would focus on two or three distributions. Ps. I am proud that I was a paid co-developer of the very first distribution listed ;).
You don't think the lessons learned by those developers' experimentation weren't shared and applied in the decision-making process for more prominent distros? In a way, creating a distro is like contributing to all the other distros at the same time. What makes them fine developers is their drive to innovate and try new things so that one day the status quo can improve. Who knows: if all those developers worked on the same few distributions, they may have crashed and burned.