Public lands can support only so many animals. The grass and habitats in Yellowstone National Park, the National Bison Range, Custer State Park, and the many other public herds can only support a certain population without damaging the land. It's a simple stocking issue. If you put too many animals in your pasture, you're soon going to exceed the capacity for the grass to restore itself. Although there is some prime bison habitat, it's unlikely that bison will be introduced into other public lands.
Most national parks are unwilling to commit the resources necessary to introduce bison.
Current national forests and national grasslands do not have the fencing necessary to keep bison in, although the National Bison Association has worked with several NBA members to swap cattle grazing permits to bison. The most opportune area for growth is private land. If you figure a 250,000 head herd, growing at between 15 and 20 percent annually, in less than ten years we will top 1,000,000 animals. With membership in the NBA growing about eight percent a year, it seems obvious that many people are building their own herd size by keeping breeding animals. When those lands reach capacity, we're going to be counting on new producers.