Can someone fill me in on why nationalism is being equated with a loss of cross-pollination, when the opposite end of the political spectrum is so strongly asso

Can someone fill me in on why nationalism is being equated with a loss of cross-pollination, when the opposite end of the political spectrum is so strongly associated with the anti-cultural-appropriation movement? It occurs to me that many of the most interesting cultures today exist almost exclusively because of they were nationalist and were not afraid to appropriate parts of other cultures into their own national identity.