For example, it wasn't but 100 years ago in the USA that groups like Irish, Germans, Italians, Chinese, etc. were looked down on as poor immigrants who were taking "real" American jobs. They eventually were accepted only to turn around and put down people who were Black, Latino, Women, etc.
Acceptance of African Americans and Women has improved a lot, and in some places Mexicans are more easily accepted than other places. However, there are still plenty of people that treat those groups poorly.
Eventually, all the current minorities, including women, will be accepted only to turn around and treat some other group like crap. Why? I assume it's because it's human nature and it's the cycle that seems to happen for things that are different. At some point we all get bullied and we all take turns bullying someone else.
It'd be nice to just skip over the ugliness, but that's not really how it works with people.
Also, let us not forget that getting treated poorly is not always a minority issue. Some people are just plain jerks and they treat everybody like crap. That's not an excuse, but no amount of positive change will get rid of jerks. They are in every industry and almost every company.