because Beneatha did not want to conform to the ways of whites
At the beginning of the scene, Beneatha is wearing the Nigerian
traditional garb that Joseph Asagai has given her as a gift, and she is
dancing to traditional music. When George arrives to take her out on a
date, Beneatha gets into an argument with him about his upper class
values which she deems as "assimilationist." By this, Beneatha means
that she thinks that George has lost touch with his ancestral African
roots and that he has conformed to American societal standards (here she
implies that these standards are racially bound and based on "white"
culture). Beneatha thinks that she is more well-rounded person because
she claims to understand her heritage.
She goes on to say that she hates ‘assimilationist Negroes’. Ruth asks
what this means and George replies, ‘it’s just a college girl’s way of
calling people Uncle Toms – but that isn’t what it means at all.’