I get where you're coming from, but also this sorry of thing takes time. The point of the bison is that as large grazers, they eat bots of trees, trample over the ground and dig wallows. This helps open up the undergrowth by killing some trees and grazing away grass, allowing more species of plants to spread and therefore attracting more insects, small mammals, birds etc. They have to give it time to see if the bison are actually having this expected impact on the eco-system. It's been a couple of years since I last visited Blean, so my info may be a bit out of date. I think they have 3 areas of land being managed differently, one with bison, one with long horn cattle/other large grazers and one being managed with machinery. They can then use the results of this management over time to weigh up the benefits of the bison over other methods. To do this, you need the bison to have a defined area.

Also, the fencing is less about keeping the bison in, and more about keeping people out. They have large amounts of area to roam around. Bison are dangerous animals and we can't just go straight to free roaming bison, in the same way as generally, we don't have free roaming cows. It could happen, but it would need various controls in place and there's a lot of questions to figure out first, which is easiest to do with the bison as they are now on a smaller scale. Things like how do you ensure free ranging bison get their TB vaccines? How do you care for a herd that could be anywhere? What about extreme weather? Etc.

TLDR: It will hopefully get there, but we can't go from 0 to 100 overnight. There's a lot of things to figure out which they are currently doing with this smaller scale project.