Notice that the greater abundance of quasars early in the Universe would be consistent with the mechanism discussed in the left panel whereby a quasar shuts off when its black hole engine has consumed the fuel available in the center of the host galaxy. We would expect that generally in the early Universe there may have been more mass easily accessible to the black hole than later, after much of it had been consumed.
Perhaps later quasars are more dependent on interactions between galaxies to disturb mass distributions and cause galaxies to begin to feed the hungry black hole.