This is my honest take- quality of football is a matter of national pride. Many countries would say their football is the best in the world. For an African-Italian to come in all "uppity" and clearly be able to outplay the best homegrown players in the league, and do so on a team with Arab owners and an Italian manager clearly hurts their pride.
Why don't they complain about the South Americans? Maybe because they look "white", or maybe because the league play in South America is generally shit, so it's understandable for the top talents to come to Europe. In fact it could be called flattering. Mario, however, they would rather stayed in Italy.