Well, it is easy:
The US tax-payers, of course, because only brain-dead politicians can undertake such stupid tasks. If people leave the rural areas, we will see how life will come back and claim the big void. However, they will be "natural" lifeforms and not the engineered scientists want.
This is the big problem, environmentalist scientists want to do two things, leave nature alone and meddle with it to give it the right shape to protect "ALL" animals.
But they have to accept that this is impossible.
Either they will start doing good science and try to use the plains as a testing ground for preservations, or they will start leaving nature alone and let animals reclaim the plains.
However, they want both and this is just impossible.
On a different approach, we can see it economically and perhaps this is the best way to see it. Economists would be against a government-sponsored wildlife project in the middle of nowhere that has no use to anybody, except unemployed scientists.
However, one could conceive it to be some kind of natural park system with special animals settled into it. In this case, private sponsors may be interested to fund and oversee the project. Meanwhile scientists can test around in the park to get the proper setting and whatever they want to do.
It would be a healthy together for scientists and private industry.
But this is as much fantasy as bringing the past back to the present.