Yellowstone bison: Why don't you just keep all the bison within Yellowstone National Park?

From the National Park Service

Why don’t you just keep all the bison within Yellowstone National Park?

While the park provides a large amount of habitat for bison, it does not provide sufficient habitat for the population during some winters when deep snow limits access to forage at higher elevations. As a result, some bison migrate to low elevation habitat outside of the park in search of food.
Annual migration allows bison to access necessary resources for their survival—similar to bighorn sheep, deer, elk, moose, and pronghorn in the system. If migration by Yellowstone bison into Montana is restricted or shortened by human intervention, then bison numbers will be largely determined by food availability inside Yellowstone, with substantial winter mortality occurring after bison reach high densities.
Fortified fences could be used to limit bison migrations, but they would also impede or serve as a barrier to the movements of other wildlife species such as bighorn sheep, deer, elk, moose, and pronghorn. Fencing creates a zoo-like atmosphere and is generally inconsistent with wildlife management principles for the State of Montana and the NPS.
The distribution of hay or commercially prepared rations at locations near the boundary of Yellowstone National Park during winter could conceivably encourage bison to terminate their migration and remain in the park. However, bison and other ungulates would become increasingly reliant on these provisions while continuing to feed on vegetation in the vicinity and degrading surrounding habitats. These outcomes are contrary to the conservation of a wild bison population and NPS policies for managing biological resources. Most natural resource managers attempt to avoid the supplemental feeding of wildlife.