identify some of the reasons why marginalized groups may not feel that allying themselves with one another was the best way to fight oppression.

NOTE- Since you asked multiple questions, we will solve the first one for you. If you want any specific question to be solved then submit the question again mentioning the question you want to be solved. 

Our society is not equal which means that some people are placed at a higher level as compared to other people who are below them. The ones who are on the lower levels of society are so because of their class, sex, race, age, income, education, nationality, ethnicity, disability, and various other deciding features. The people who are in the low runs of society are marginalized from mainstream society and treated as minorities. Since the dawn of history, we have seen instances of marginalized people getting mobilized and fighting against injustice and this has given them great success as well.