I promise I'm not being sarcastic..... but....
Does it matter?
Boeing designed the aircraft - they say keep the speed under 260kt under 6000ft, so probably they have a good reason for saying that!
The FAA/EASA etc certified the aircraft and the checklists.
Your operator pays you to operate IAW the Ops manual.
Most of the world has a 250kt below 10000ft restriction.
Simples
But yes I am sure there are interesting technical reasons.... and probably Fly3 is correct