Because the Nazis appropriated the symbol from ancient eastern religions.
"The swastika has an extensive history. It was used at least 5,000 years before Adolf Hitler designed the Nazi flag. The word swastika comes from the Sanskrit svastika, which means “good fortune” or “well-being." The motif (a hooked cross) appears to have first been used in Neolithic Eurasia, perhaps representing the movement of the sun through the sky. To this day it is a sacred symbol in Hinduism, Buddhism, Jainism, and Odinism. It is a common sight on temples or houses in India or Indonesia. Swastikas also have an ancient history in Europe, appearing on artifacts from pre-Christian European cultures." from the Holocaust Museum website.