Here's why:
* Different species: Musk oxen (Ovibos moschatus) belong to the Bovidae family, but are in the genus *Ovibos*. Bison (Bison bison) are also in the Bovidae family, but in the genus *Bison*.
* Genetic differences: While they share a common ancestor, their genetic material has diverged enough that their chromosomes are not compatible for fertilization.
* Physical differences: Musk oxen and bison have significant physical differences, including size, horns, and coat. These differences further contribute to their reproductive incompatibility.
Even if a hybrid offspring were somehow produced, it would likely be infertile.
So, while they are both large, herbivorous mammals in the Bovidae family, musk oxen and bison are separate species that cannot interbreed.