(1) Donkeys are already domesticated. More species that can be used = more options.
(2) The donkey was available in the 19/β20th 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.
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/β20th 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.