The stakeholders in this matter are primarily third-sector organizations in support of beaver reintroduction, of which the many regional Wildlife Trusts are most prominent, and farmers, who are generally but not entirely against reintroduction.

The licences are necessary because with such a sensitive issue, it must be ensured that only responsible and well-resourced organizations can release beavers into the wild. Otherwise, beavers may be introduced into areas that are unsuitable for them, or into areas where they will cause disproportionate damage to farming operations, and either eventuality would harm the perception of beavers. If that happened, the beavers might become so unpopular that it becomes politically impossible to continue with the reintroduction, so it's in everyone's best interests to be careful. Beavers are wonderful animals and can be very beneficial to their local environment, but you can't just dump them on a housing estate or something and expect good results.

I don't know what the exact criteria is, but the winding, damp brooks of the area where I live have been among the successful trial areas for beaver reintroduction. An abundance of willow trees is one factor that makes for an ideal beaver environment, for instance.