Why hasn't the principle of equality always been adhered to in our democracy

Because people often thought of white men to be more superior to black men and therefore treated them unfairly. Also, people thought of women as being a wife and a mother and to stay at home and do housework. This led to women and blacks people standing up to what it right which white men then disagreed with and that is when it all started as there was a lot of conflict between them

Although the constitution claimed all men are created equal there was an unwritten bit that was agreed upon by everyone that the constitution only referred to all white men, and most people of the time considered women to be housewives who had no business in politics, jobs, or dealing with money.

this is just going off the top of my uneducated knoggin.