Shaft drive delivers power better, wears less than a chain, easier to maintain.

Swings, roundabouts, and tangential from answering why "Torque, electric motors need gearboxes" doesn't answer the progenitor question "why chain".

My bike vendor likes chains. They wear more easily, but are easy enough to maintain and change. Basically everything old school bike like has maintenance/repair advantages.

I'm not sure I agree, but more when compared with belt drive and internally geared hub. But then again, I believe him that those are hard to repair when they fail (even if they are sturdy until then).

I also prefer shaft drive over a chain. Much less hassle and if not punished, can be almost maintenance free. A chain is easily worn and can even rust away just sitting there.

[delayed]