Narcissists often target people who have been abused before or people who have a poor support system. They know that people who have a lot of concerned friends and family members are more likely to be protected and supported, as well as helped and advised during an abusive relationship.
The narcissist wants to turn you against, or at least separate you from anyone who can help you or see that something is wrong. It is easier for a narcissist to brainwash you, abuse you and control you when there is no one looking out for you and you have nowhere to turn. They like to poison relationships in general so they can have all of your attention and use all of their power over you.