Because nothing happens now that wasn't rooted in then. Because what happened to who, when it happened and why, are central to understanding the what and why of now. Because you can't change anything until you know why it is the way it is.
One of the reasons racism persists is because, culturally, white Americans are trained to see it as an individual problem - an issue of prejudiced individuals, not a racist system. They are acculturated to reject any notions of collectivity - collective responsibility, collective action. Since systemic racism is a collective responsibility, they are unable to recognise it, much less deal with it.
Which is why I've had to spend half my life working against individuals, for the good of individuals, case by case. You can't fight a system which everybody denies exists, so you do what you can.