Why didn't the US Declaration of Independence really protect human rights even when it stated “Life, liberty and the pursuit of happiness” as its goal?

Essentially because the declaration of independence only included white people.

Explanation:

The United States only protected the life, liberty and pursuit of happiness of white people in its early history, since Native American populations were subject to poor treatment, and their lands were expropriated, while African Americans were in even worse conditions since they themselves were considered property that could be bought and sold as slaves.

These issues were only resolved with time, first after the abolition of slavery and the recognition of native American lands, and then, with the civil rights movement and civil rights act.

i'll just take the points xd