As soon as the population gets large enough, people who want to hunt them for sport will take over, with the help of lobbying groups and members of Congress. We also do not have enough grazing land available for bison to hit the levels they were at prior to the 1800's.
The amount of available farmland which has been reduced by urbanization and corporate farming has severely reduced the amount of grassland where the bison population of pre-1800 can graze. The lands have been consumed for large scale farms to produce food and energy grains for the human population. There is hardly enough land for current wild animals which are being driven back and out of their natural habitats.
Before 1800, there weren't so many people living in America and now there is. So the bison don't have that much space for their habitat. This alone will prevent the bison levels from going back up to what they used to be. Plus, there has been so much development, they probably won't adjust to life in this current environment if their numbers go up.
It's highly unlikely that the bison population will ever return to its pre-1800 levels, because the animals are killed for their skins, and basic logic dictates that the bigger the population of people, the lower the population of animals, because their habitats are constantly being destroyed. Unless a concerted effort is made to preserve and encourage the bison population, it will never be as high as it was.