You make an excellent argument and I apologize for selling the wisent short. It really is silly to argue which is the more historically significant animal. Still, I have no doubt that the American bison is the more popular animal and would be prioritized over the wisent almost assuredly. Both should indeed be in the game, but the wisent just screams "DLC animal" to me.The wisent meanwhile has one of the historically most important conservation efforts as it simply was one of the very first and arguably most successfull.
It shaped the way ex situ conservation has been done, which makes it in a zoo context far more historically significant then the american bison.
They are also among the creatures most commonly depicted in cave paintings and other prehistoric arts and craft, with one of the oldest artistic object found, dating 40.000 years ago, being the small figure of a wisent.
Both have their place and both are significant in their own way so Statements like the implied larger historicy significance of the american bison most of the time are more backed by the lack of knowledge over the other creature then the knowledge of the one presented.