People are quick to anger because of the language barrier.
They're expensive to conduct.
Different cultures are too dissimilar to achieve meaningful results.
It is difficult to ensure that the same variable is being studied in each of the cultures.
People's prejudice interferes with the studies.
It is possible to control the animal's enviromment to isolate a variable for study.
Animals are less expensive to work with.
Animals have simpler behaviour that is easier to model.