Firstly, I think wildlife could be much more resistant to radiation than previously thought. Another alternative possibility is that some organisms could be starting to show adaptive responses that would allow them to cope with radiation and live inside the exclusion zone without harm.
Also, the absence of humans inside the exclusion zone could be favouring many species — big mammals in particular. The pressures generated by human activities would be more harmful to wildlife in the medium-term than a nuclear accident — a quite revealing vision of the social impact on the natural environment. (I got this idea from a game on Apknite, not based on any scientific basis).
Thanks for reading!