I think, as others have mentioned, there is an implicit ethnonationalism here in linking various governments, specifically post-colonial states (say Nigeria) with the tribes that live in them. Now, its important to note two things here. 1) many of these tribes, before colonialism, before the establishment of the contemporary west-african states, were in fact slave empires, and the reason they had such rich cultural products was for the same reason that western nations, at the time, did. So the moral status of those objects is ambiguous at best, which leads to point 2) that these objects should be stored in whatever facility can host them best. We all admit Nigeria is not as wealthy as the UK, so its entirely possible that the UK after returning perhaps, thousands of precious artifacts, worth an uncountable sum, a desperate government might, given the right circumstances, sell them, whereas the UK probably will not (at least not in the near future).
Call me conservative if you like, but culturally important objects are often only designated as such the moment they are placed in a museum. Who knows if the former slave empire tribes of west Africa would've valued all the pieces so highly had they not been put on display. By "getting them back," these ethnic groups are merely reproducing the same order that they oppose, in fact they have completed, in a way, the colonial circuit, by becoming a mirror of the power that created them.
Call me conservative if you like, but culturally important objects are often only designated as such the moment they are placed in a museum. Who knows if the former slave empire tribes of west Africa would've valued all the pieces so highly had they not been put on display. By "getting them back," these ethnic groups are merely reproducing the same order that they oppose, in fact they have completed, in a way, the colonial circuit, by becoming a mirror of the power that created them.