Isn't part of the problem with the wisent that it is a browser instead of a graser and thus harder to feed?the wisent did though, right next to the aurochs. Which makes you wonder if the aurochs, in spite of it's size, was just easier somehow to domesticate than the wisent or bison (which are very closely related)…
Also about the cape buffalo, I believe it is often considered one of the most dangerous animals in Africa, as in it is often considered more dangerous than lions and such. Not an easy animal to domesticate, especialy if there are better choices.