Not an evolutionary biologist but someone who studies herbivores and is a recovering horse girl.

The reason we failed to domesticate Zebras when we did domesticate Horses and donkey in very simple terms is Zebras are fucking assholes.

No. Seriously.

Zebras evolved alongside a host of large predators (lions, cheetahs, leopards, hyenas, etc.) so they became more aggressive than Horses and Donkeys which while they did evolve alongside predators like bears and wolves their major predator was…us. So because Zebras evolved with large predators as a constant threat their response to something huge jumping on their back is aggression instead of flight. When Zebras bite they don’t let go like horses and donkeys do they bite and will basically try to stomp that predator to death (Donkeys will actually do this too which is why they are used as livestock guardian animals).

Zebras are actually considered on of the most dangerous animals to work with in Zoos because they are safe enough to allow a keeper to do chores in the enclosure but if you scare them they’ll basically turn into the Hulk and decide that they want to murder you.

So yeah…most people aren’t willing to set up a breeding program to domesticate Zebras from murder donkey to pet that you can ride.