Answer :
They were susceptible to the plague because it was a new disease to people. Their system didn't have the antibodies to fight off the sickness. Also, they were living in pretty bad conditions so the disease was easily spread.
Not only was it a new disease (therefore their bodies weren't resistant to it), but the Europeans are known (historically) for not being the cleanliest. At the time of the plague, dirt and germs, as well as a lack of personal hygene, made Europeans very suceptible to the plague.