Basically the Nigerian federal government, the state government of Benin, and the descendants of the kings of Benin who originally owned the stolen Benin bronzes all think they should be the ones to whom the bronzes should be returned -- and in the case of the current king, he asserts they are his family's private property.
On top of that, there is a long track record of art being stolen from modern-day Nigerian museums and sold to other museums or collectors, and returning the bronzes just for them to be re-stolen does not benefit any of the parties involved.