The best way to find out WHY would to be educating yourself on both sides. Ask the Native Americans their side of what happened, than compare notes on both sides.
As for this being a Geneology forum,Peoples ancestors were a part of the Battle at Wounded Knee, so not only was it history, but the life and death of our ancestors. I want to know more about my ancestors than just a name and dates. I want to know how they lived between the dash (Birth-death dates)Granted I may not find out, but a part of the History might explain some things.