From the National Bison Association site's FAQs:

Any product labeled as buffalo milk, or buffalo cheese is produced from the milk of water buffalo. Water buffalo are a separate species, and are not related to bison, even though American bison are commonly known as buffalo. Bison are not milked commercially for a couple of reasons. First, the teats on female bison are very small. Also, bison are undomesticated animals, and the females do not adapt well to the type of handling necessary in a milking operation.

I imagine that might be quite something to see: "the females do not adapt well to the type of handling necessary in a milking operation".