György Buzsáki (1949--) The Brain from Inside Out (Oxford University Press, 2019)
Chapter 11. Everything Is a Relationship: The Nonegalitarian, Log-Scaled Brain
...We can only speculate how skewed distributions emerge in the first place. During neurogenesis, early-born neurons have a higher chance of innervating each other, and the synapses they make on the budding dendrites are placed nearest to the soma. Thus later-born neurons can only innervate the more distant segments of the dendrites. Based on this observation, we can conjecture that early-born neurons are more effective in discharging their peers, so the firing rate distribution may have a simple chronological origin.
...We can only speculate how skewed distributions emerge in the first place. During neurogenesis, early-born neurons have a higher chance of innervating each other, and the synapses they make on the budding dendrites are placed nearest to the soma. Thus later-born neurons can only innervate the more distant segments of the dendrites. Based on this observation, we can conjecture that early-born neurons are more effective in discharging their peers, so the firing rate distribution may have a simple chronological origin.