Because Dead Rear was deemed politically incorrect.
Actually because of the Red Deer River which runs through it. When the first white settlers were showing up in this part of the world they didn't know what an elk was, so they called them Red Deer instead because they look similar and they knew what that was. So the was-ka-soo (elk in cree) river became the Red Deer river.
Wow, I guess I did learn something in school when I was little.