(1) Donkeys are already domesticated. More species that can be used = more options.
(2) The donkey was available in the 19/20th centuries and yet people aimed to domesticate zebras then. Various reasons, I suspect: disease resistance, speed, perhaps strength. There have been roles for both donkeys and horses in places that have both.
Why zebras rather than donkeys, which are native to Africa and whose population is increasing there? https://thehorse.com/features/beasts-of-burden-africas-working-horses-and-donkeys/
Good question. Couple of thoughts:
(1) Donkeys are already domesticated. More species that can be used = more options.
(2) The donkey was available in the 19/20th centuries and yet people aimed to domesticate zebras then. Various reasons, I suspect: disease resistance, speed, perhaps strength. There have been roles for both donkeys and horses in places that have both.