So many people died because of how fast it spread and what it did to the body. The black death spread so fast that all you had to do to get it would be to just get somewhat close to someone who was infected, and the black death killed by causing hemorrhages inside the body that made your skin turn black because of how much blood you lost from internal bleeding.
So many people died from the black death because it was so contagious and easily spread. Anyone who came in contact with a person who had even came in contact with a flea that had the virus would be infected. The disease was spread airborne as well. Thus, causing so many deaths.