Do you think politics is the only reason Beneatha declares she hates assimilationists? If not, what could another factor be?

Beneatha does indeed have strong political reasons for hating assimilationists. She sees them as sell-outs who've given up their own unique heritage to submerge themselves in the dominant, oppressive culture. That's why she embraces Yoruba culture, wearing Yoruba clothes and having her hair styled like a traditional Yoruba woman.
But the personal is the political, as they say, and there are clearly equally strong personal reasons why Beneatha is so hostile towards assimilation. Putting on traditional Yoruba clothing and getting in touch with her African ancestry gives her a great deal of confidence and self-respect. This, more than anything, is what she really needs in her life right now. Her dreams of medical school appear to have gone up in smoke after the insurance money that was going to pay for her education is stolen. So at this emotionally vulnerable moment in her life, she desperately needs something to hope for in future. And such future hope is provided by a reconnection with her ancestral past.